#help-13

428200 messages · Page 464 of 429

south tundra
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Well actually you could use it again

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Let me show you

late spoke
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alright, please

south tundra
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So the 2nd sum is (n + 1) + (n + 2) + (n + 3) + ... + 49, right?

late spoke
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aye

south tundra
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You could rewrite this as (n + 1) + (n + 1 + 1) + (n + 1 + 2) + ... + (n + 1 - n + 48)

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But this is the same as 0 + 1 + 2 + ... + (48 - n) + n*(n + 1)

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What I did here was take out n+1 from every term of the sum

late spoke
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i see...

south tundra
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Oh wait I think it should be (48 - n)(n + 1) there

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Because in the sum there are n - 48 terms

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Wait let me think for a moment

late spoke
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ok

south tundra
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Yeah so the sum is now 1 + 2 + ... + (48 - n) + (48 - n)(n + 1)

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We could use the formula here to rewrite this as (48 - n)(49 - n)/2 + (48 - n)(n + 1) I guess

late spoke
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i got something like thisd

south tundra
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Is that product of (48 - n), (n - 1), (48 - n) and n divided by 2?

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Btw I messed up a bit there, should be (49 - n)(n + 1)

late spoke
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this hw is stressing me out

south tundra
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Yeah so the 2nd sum is basically this

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Now all we need to do is set 1st and 2nd sums equal to each other

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So what we have is this

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The rest is solving for n

late spoke
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i see

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ok, ill see what u can get from here

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also sorry to ask u for this, but can u stay here please? i got 2 other questions that im stuck on as well

south tundra
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Surely

late spoke
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thank u a ton

south tundra
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Might get a bit busy soon but I'll try to respond anyways

late spoke
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its part 2

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i asked others in the other hw help qu

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but they didnt understand

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first of, this isnt an infinite series

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its finite to the term of n

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second, x is not less that one in this case

jaunty mural
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differentiation comes to my mind

late spoke
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hold on, lemme get the paper out, its like the 3rd time i tried to redo this question

jaunty mural
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it just seems like it to me but idk

late spoke
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something like this

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but i keep screwing it up somewhere

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nvm

jaunty mural
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wut

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1 +x +x^2 + x^3 +...

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this is geometric

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idk how u got your last line

late spoke
jaunty mural
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look this thing is geometric series

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write formula

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everything before last line is ok

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oops

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its geometric plus an extra last term

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$$1 +x +x^2 + x^3 +... + x^{n-1} -nx^n$$

wraith daggerBOT
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Shuri2060

jaunty mural
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I believe, but check.

late spoke
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got the same as well

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...ignore what i wrote on top

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ok... i think i got a solution

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imma check, by plugging in values x=2 and n=4, i should get 49

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i got it....

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I GOT IT

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AAAAAHHAHAHHAHA

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FINALLY

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@jaunty mural @south tundra THANK U, NOW SEQUENCES CAN GO KISS MY ASS

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HW FINISHEDD

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AAAAAAAHHAHAA

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THANK U AGAIN

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.close

cedar kilnBOT
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crimson sedge
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hi, i need help

cedar kilnBOT
crimson sedge
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sorry for the bad quality pic

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<@&286206848099549185>

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<@&286206848099549185>

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You would find the height by letting t = 0

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height of the bridge?

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Yes, because the rock falls from the bridge. So at the beginning, the height of the rock is the height of the bridge

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oh okay

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How would you find the maximum height of the rock? You could try sketching the quadratic equation

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wait so the answer for a is 15 right?

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then how is the rock at 25?

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Set h = 25 and solve for t

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Is it possible? Can you get any answer?

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nope

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didnt work

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Yes, and what did you find as the maximum height?

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So for d) you could write 'never'

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is that a thing

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i think we are doing something wrong

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<@&286206848099549185>

crimson sedge
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I finally understood the whole thing. Thank you!

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@cedar kiln

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.close

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deft spruce
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Is the point of inflection on a cubic graph also the midpoint between the minimum and maximum points of the curve?

cedar kilnBOT
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@deft spruce Has your question been resolved?

jaunty mural
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Not at all.

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Take $x^3+x$

wraith daggerBOT
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Shuri2060

cedar kilnBOT
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@deft spruce Has your question been resolved?

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humble ledge
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Hi, I'm a bit stuck on trying to figure out if a certain sets of datapoints are generated by a discrete uniform random number generator. The cases I'm having the most trouble with is cases where there are a low amount of samples, or a high amount of possible output values. Ideally I would like to calculate a p value to see if I should reject the null hypothesis that i'm dealing with a discrete uniform generator. I did realize that the amount of times a certain value is hit is distributed according to a binomial distribution, however I am a bit unclear how I should use this to test my null hypothesis.

humble ledge
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An example of a data set (at least the counts of the elements)
62 48 59 70

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(searching for tests that confirm or reject uniformness all lead me to pages about the uniformity of p values)

steady imp
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What’s your alternative hypothesis

humble ledge
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so for a binomial distribution with n = 239 and p=1/4 the chance that 48 or smaller comes out is 4.4%

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that it isn't uniform?

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I don't know if I can make a better alternative hypothesis

steady imp
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It’s more like

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How do you even test that

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So like you could fit it?

humble ledge
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well, a alternative is that it is correlated to other attributes of the datapoint

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like if I only look at the data points that also have a certain attribute, I get the counts 11 10 13 26, which my guts say is not uniform, but I can't really prove it

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I have one other idea

steady imp
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I'm looking it up right now

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trying to find something mathematical

humble ledge
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one way of defining a p value would be:
if I generate data sets according to my null hypothesis, what is the probability that my given sample lies further away from 1/4,1/4,1/4,1/4 then this random sample according to a given distance metric

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I think that should use the taxicab metric

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as then equally likely situations lie equally far (I think)

steady imp
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So yeah it would be like

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something like "there's a p % probability for this particular datum to be situated where it is"

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But I'm trying to find a real method fo rit

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that for every data point

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The goodness of fit of a statistical model describes how well it fits a set of observations. Measures of goodness of fit typically summarize the discrepancy between observed values and the values expected under the model in question. Such measures can be used in statistical hypothesis testing, e.g. to test for normality of residuals, to test whe...

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This has it fully laid out

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So you're basically making a histogram like this

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You're histogramming your data into "bins" and comparing the size of each bin to the expected result

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and then you should get a statistical answer

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Does this make sense?

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I think making your number of samples the same number of data points might be good

humble ledge
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euh

steady imp
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In that case given your data, your value should be $\chi ^2 = \sum_{i=0}^n{(O_i-1)^2}$ where $O_i$ is the number of data points within the region

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Er, that's wrong

wraith daggerBOT
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KurtDee

humble ledge
steady imp
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What's your max

humble ledge
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what do you mean max?

steady imp
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Like you're sampling uniformly, but like from what uniform distirbution

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0-100?

humble ledge
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0-3

steady imp
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11 is greater than 3

humble ledge
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yeah, those are the counts

steady imp
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OH

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oh this is easy then

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You already have a histogram lmao

humble ledge
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right, so chi squared would be the euclidean distance (squared) to [1/4, 1/4, 1/4, 1/4] * n

steady imp
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I'm not sure I understand

humble ledge
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to be honest, I think using taxicab distance is more correct

steady imp
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I'm not sure what this distance is

humble ledge
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thanks

steady imp
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that seems right to me

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I hope you understand the math

humble ledge
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Yeah

steady imp
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I was just doing it in excel lmao

humble ledge
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I think the calculator I used used two tail

steady imp
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Two tailed makes sense

cedar kilnBOT
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@humble ledge Has your question been resolved?

deft spruce
cedar kilnBOT
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humble ledge
deft spruce
humble ledge
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The inflection point is the max/min of the quadratic differential which is in the middle of the 2 zero points, which are the min and max

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@deft spruce also interesting: a cubic function is point symmetric around the inflection point

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Wait is it

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I'm not sure anymore

deft spruce
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Wouldn't that imply that the concave and convex part of the graph were identical in size?

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OK, and apparently that is consistently true across graphs?
"The graph of a cubic function is symmetric with respect to its inflection point"

humble ledge
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Yeah, seems like I remembered it correctly

deft spruce
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Graphs are cool

cedar kilnBOT
#

@humble ledge Has your question been resolved?

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solemn heron
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do you know what the argument of a complex number is_

solemn heron
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?

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i ask bc i speak another language and i want to make sure its the same term

drifting matrix
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the intuitiv definition is that it is the angle between the positive real axis and the number.

solemn heron
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oh yeah its that one

drifting matrix
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i.e. arg(i) = pi/2, arg(1) = 0, arg(-i) = 3pi/2

jaunty mural
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The anticlockwise angle from the positive real axis

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The range it can be depends on definition

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-pi to pi or
0 to 2pi

solemn heron
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-1 +√3 i ..... i need to get it from this complex number

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but when i do the math it gives me -60°

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and it should be 120°

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the argument

drifting matrix
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yeah that is different

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even if you accout for what Shuri said.

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Can you make a sketch?

solemn heron
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with the cuadrants?

drifting matrix
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yes

solemn heron
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i have it

drifting matrix
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If you sketch it should be clear that 120° is about right

jaunty mural
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If you're working with complex numbers, you really should use radians not degrees

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just going to say

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You can think in degrees, that's fine.

solemn heron
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yeah im using radiants but im not very good with them. its easier to ask in degrees

jaunty mural
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Anyways

solemn heron
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but yeah thanks

jaunty mural
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For the math i dont see how u get that answer

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u should sketch on argand diagram

solemn heron
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i pass it to radiants afte

jaunty mural
drifting matrix
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yes

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so more than 90° aka pi/2 and less than 180 aka pi

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Actually I think it is always helpful to sketch this or imagine it how it looks

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that is how I figure out the right range anyway.

solemn heron
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and how do i calculate it?

jaunty mural
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You should work out x

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between 0 and 90

solemn heron
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lets say if i cant see it in the sketch

jaunty mural
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then subtract from 180

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???

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Of course you have to sketch

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Not sketching is bad

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Sketch in head

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or sketch on paper, doesn't matter.

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I would never do it without.

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In particular, you need to know which quadrant the number is in, otherwise you will fail to work out the angle.

drifting matrix
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You could memorize the cases table but it's not a good idea imho.

solemn heron
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its the first and forth quadrant is the same

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and the second and third are the same

drifting matrix
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the same?

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how do you mean that?

jaunty mural
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its not.

solemn heron
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i mean in the first quadrant both imaginary and real are positivw

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and the forth both are negative

drifting matrix
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nooo

jaunty mural
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I don't think we agree on the quadrant labelling

solemn heron
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so the process of getting the argument is the same

jaunty mural
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Standard is to count anticlockwise

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Just like your angle.

solemn heron
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yeah yeah, thats how i see it

jaunty mural
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The process isn't the same

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or if it is

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I strongly suggest you don't consider it to be

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Just get used to quickly sketching an argand diagram

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This takes like 5 secs on paper

solemn heron
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so its only like a visual thing to calculate it?

jaunty mural
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yes.

solemn heron
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oh okay

jaunty mural
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I can see the angle between 0 and 90 i need to calculate

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and then I see if I add? subtract?

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to 0, 90, 180, 270, 360

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whatever it is

solemn heron
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yes, i see what you mean

drifting matrix
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One thing that is maybe worth memorizing is that if it is only at the right side (quadrants 1 and 4), then you can calculate it as atan(imaginary / real)

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keep in mind though that it needs adjustment when it is not at the left side.

solemn heron
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yeah thats how i was doing it

drifting matrix
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aha

solemn heron
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but when i do it like that it doesnt give me the 120°

drifting matrix
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yeah because you are at the left side

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as you can see

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imaginary / real

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gives the same answer

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if you negate both.

solemn heron
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and to do that i just do √3/-1

drifting matrix
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If it is at the right side you would need to add or subtract 180° aka pi (both are fine depending what definition you prefer)

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I suggest to follow Shuri's advice though to sketch also.

solemn heron
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yeah the thing is i need both

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the sketch and the process

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and i cant finish understanding the process

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i know i should do arctg (√3/-1)

solemn heron
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but im not sure how to solve it

drifting matrix
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Unfortunate it does not embed the image

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it shows the table of cases if you wanted to figure it out without any sketching at all.

solemn heron
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oh yeah thanks a lot

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🙂

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thank you very much

drifting matrix
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you are welcome

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I think this is hard to memorize though without at least sketching in your head.

solemn heron
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no yeah i know, i always do the sketch first

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but if i have to do an exam of this i need to write how i got it

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thats why i cant just do the sketch

drifting matrix
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Hm if I was the teacher I would accept that obviously

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but if it does not work for you, oh well

solemn heron
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hopefully they dont bother much about it

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but you are never sure at uni haha

drifting matrix
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btw I think neither me nor Shuri meant ONLY sketch

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we were talking about sketching as a help to figure out the right case.

solemn heron
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yeah i always sketch

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and its rather easy to see how much is the argument

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its just that i dont get the math of it

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when i do it myself

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and i clearly dont know how to use the calculator bc im not getting anything close to that °

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120°

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but thanks a lot, its really nice that you guys are helping peoplle

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ill close it now, have a great day 🙂

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.close

cedar kilnBOT
#
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turbid needle
#

I need help determining linear independence, I understand that its making a linear combination of the vectors equaling the zero vector but i'm not sure y i keep getting it wrong.

turbid needle
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rn i just think A, C, and F are Linear independent <@&286206848099549185>

brave aspen
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You need to stick the vectors into a matrix equation set equal to the other vector(s)***

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And solve

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If there is only one solution the vectors are linearly independent

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Other pointers

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Any set with zero vector is NEVER linearly independent

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Even just the Zero vector by itself

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So A is not Lin Ind

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Also a set that has more vectors than entries in the vector cannot be linearly independent

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E.g. set of 3 vectors with only 2 values in them

turbid needle
brave aspen
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Correct

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So if you have only 2 vectors

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You just try to find a scalar constant that multiplies one into the other

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If you cannot find such a number, then those are independent

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E.g.

(1,2) ; (3,3)

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There is no constant that can turn one into the other

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I can solve it algebraically out by entries to be sure but i think you should see that one intuitively

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But take a look at

(1,2) , (2,4)

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Multiplying the first one by scalar value of 2 gives (2,4)

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These are not independent

turbid needle
brave aspen
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Well if its only 2 then yes

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But if you have more than 2 it becomes more complex

turbid needle
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oh

brave aspen
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Because e.g.

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(1,2), (2,1), (3,3)

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Any 2 of those are independent wrt each other

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But if you take

(1,2) + (2,1)

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You create (3,3)

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Adding different multiples of vectors is what we call a linear combination pf those vectors

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The real definition of linear independence relies on linear combinations of all the vectors

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So if i can add different multiples of vectors together to create one of the others in the set

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Then you do not have independence

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In order to check for sonething like D.

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You need to use a matrix equation

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First determine if any 2 of the vectors are independent

turbid needle
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my work on D

brave aspen
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This is how i would approach it

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Well

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Actually your way is fine

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For 3 vectors with 3 entries

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When you have as many vectors as you do entries, you use the homogenous equation you setup

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That equation has only one solution if and only if the vectors were linearly independent

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But personally i would stick the first 2 vectors in a matrix and solve for the 3rd vector

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If there is no solution, then you have linear independence

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I prefer that way of thinking because it works for any size set of any type of vector

turbid needle
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ok so is C LI since its one vector

brave aspen
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Yes

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G. Is another good example

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Definitely NOT LI

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Because more vectors than entries in the vector

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(Which means more vectors than there are dimensions in that vector space)

turbid needle
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because the space is Real^2 and there are 3 vectors?

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ok i think i understand the content

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ty i will answer these now

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.close

cedar kilnBOT
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glossy root
cedar kilnBOT
glossy root
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hi so

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I have to use gauss elimination to calculate the inverse matrix

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and at the end I get the correct answer but with 5s instead of 1s on the left side?

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is this still correct?

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any help would be appreciated

drifting matrix
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,wolf {{-1,7},{-2,9}}^-1

glossy root
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ohh

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I have to divide with 5

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bro

drifting matrix
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Yeah it seems the calculation is correct

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until the very last result

glossy root
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yeh

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mb

cedar kilnBOT
#

@glossy root Has your question been resolved?

cedar kilnBOT
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gentle ore
#

i solved this question but idk where i went wrong

tight herald
#

"I solved.. idk where I went wrong"

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??

gentle ore
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i "solved" but got wrong answer

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so ig i didnt acc solve

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lol

tight herald
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Oh..

dense wing
gentle ore
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idfk what i’m doing lmao

dense wing
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you did all the hard stuff fine

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you have a slope (1/6)

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and a point (8,3)

gentle ore
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o i just did line eqn wrong

dense wing
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yep

gentle ore
#

alr ty lol

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.close

cedar kilnBOT
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fair narwhal
#

guys does anyone know how to begin proving this is true for all xeR?

drifting matrix
#

What tools do you have available?

fair narwhal
#

what do you mean by tools?

dawn jetty
#

perhaps sin(a+b) identity would be a good start

fair narwhal
#

i tried that but it seems useless

drifting matrix
#

One tool that is incredibly powerful with this kind of question is to use the complex number exp identity but I am not sure if you are allowed to use that.

jaunty mural
fair narwhal
#

you mean sin(x + 2x)?

drifting matrix
#

sin(x) = 1/(2i) (exp(ix) - exp(-ix))

dawn jetty
#

it's the most intuitive thing to get sin3x in terms of x

drifting matrix
#

Using this single identity plus usual exponentail stuff I am pretty sure one can do it easily.

#

But I don't wantto confuse you with complex numbers if you haven't seen them.

fair narwhal
#

no its all good ive heard about them

jaunty mural
#

Just basic compound angle identity is enough.

#

+common trig identities

fair narwhal
#

thanks for the ideas guys

#

im gonna try again

#

.close

cedar kilnBOT
#
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cedar kilnBOT
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fair narwhal
#

guys how can i know if this series converges?

west dome
#

what tests do you know

#

for convergence

#

actually, can you post the full problem first

fair narwhal
#

i know

#

the leibniz criterion?

west dome
#

this is the test you want to use, yeah

fair narwhal
#

quotient criterion

west dome
#

but can you post the entire question first?

#

if n is finite then this is trivial

fair narwhal
#

umm

#

its in german though

west dome
#

thats fine

fair narwhal
west dome
#

Okay, so its actually asking you if the sequence (x_n) converges, not if that series in particular converges

fair narwhal
#

ive tried the ratio test but it seems to be 1

#

so if that inside the sum converges?

west dome
#

this is equivalent to asking if the infinite series converges. It is interesting that they didnt just ask you that outright

fair narwhal
#

so with the sum?

west dome
#

Equivalently, the question is asking if

#

$\sum_{k=1}^\infty (-1)^k \frac{1}{\sqrt{k}}$

wraith daggerBOT
#

Migillope

west dome
#

converges

#

Now you can use the Leibnitz criterion (aka alternating series test)

fair narwhal
#

so this means sum from n->inf ?

drifting matrix
#

If there is a (-1)^n thing in it your alarm bells for the Leibniz criterium should always start ringing.

west dome
#

No, thats the sequence of each of those sums with upper bound n

#

so the first term is $\sum_{k=1}^1 (-1)^k \frac{1}{\sqrt{k}}$

wraith daggerBOT
#

Migillope

west dome
#

the second is $\sum_{k=1}^2 (-1)^k \frac{1}{\sqrt{k}}$

wraith daggerBOT
#

Migillope

west dome
#

etc

#

so as n gets really really big, itll get really close to $\sum_{k=1}^\infty (-1)^k \frac{1}{\sqrt{k}}$

wraith daggerBOT
#

Migillope

fair narwhal
#

okay i see now

west dome
#

which is why if we can show $\sum_{k=1}^\infty (-1)^k \frac{1}{\sqrt{k}}$ converges, we're done

wraith daggerBOT
#

Migillope

fair narwhal
#

its so difficult because there are so many criteria

west dome
#

yeah, but as @drifting matrix said, if you see a (-1)^k in the series, you should immediately use leibnitz criterion

#

(I prefer the name alternating series test because it makes it a lot easier to remember what it actually does)

fair narwhal
#

i see

west dome
#

anyway, do you see how to solve this using the leibnitz criterion?

fair narwhal
#

so this would be alternating too?

west dome
#

no, there is no (-1)^k here

fair narwhal
#

oh i thought because of cos()

#

okay but youre right I'll have to try to solve a) first

west dome
#

no matter what power you raise e to, it will never be negative

#

so there is no alternating ness

#

although, you might want to examine part b by saying "well if the exponent converges, then the sequence converges"

#

then you can probably use alternating series test

#

but we can cross that bridge when we get done with a)

fair narwhal
#

ok thank you very much

west dome
#

ping me when you are done (or stuck at some point) on a

fair narwhal
#

.close

cedar kilnBOT
#
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fair narwhal
#

@west dome hey i think I did it:
first: i set 1/sqrt(k) as my x_n
second: i did the lim of x_n, which was 0.
third: i proved if x_n >= x_n+1, which was true

west dome
#

Looks like the right process!

#

You ready for b)?

fair narwhal
#

but im not sure how that proved a)

west dome
#

oop

fair narwhal
#

is it possible to find the actual value it converges to, or would that be too difficult?

west dome
#

you could. my german is rusty, but the question just asks if it does converge, right?

fair narwhal
#

yep

#

Okay i guess im just gonna trust these 3 steps

drifting matrix
#

I think finding the limit would be difficult in this case.

#

,wolf sum (-1)^n/sqrt(n), 0<n<oo

drifting matrix
#

basically wolfram alpha is cheating

#

gamma function is just defined like this.

west dome
#

They're trustworthy steps, seeing as how it is the alternating series test, which isnt too hard to prove!

fair narwhal
#

Alright

west dome
#

so @fair narwhal did you want to do part b?

fair narwhal
#

yes

west dome
#

ok, do you have anny ideas

fair narwhal
#

there is another tip for this i think: if no limit exists, check if an accumulation point exists

fair narwhal
#

i can maybe just use the cos pi sqrt(n)?

west dome
#

yeah so we have the condition that exp(cos(...)) converges iff cos(...) converges

#

so: you need to test if cos(...) converges

fair narwhal
#

ah but cos never does

west dome
#

cosine never does, true, but cos(1/n) surely does, right?

#

for example

fair narwhal
#

Ok true

#

to either 1 or -1 right?

west dome
#

to 1

#

since 1/n -> 0 as n-> infinity and cos(0)=1

#

so, you need to look closely at pi*sqrt(n)

#

does this converge?

fair narwhal
#

nope

#

but i dont understand, which criteria should be used now?

west dome
#

uh... logic. I'm not sure the name of the criterion, I can check if youd like

#

Divergence test

#

there it is

fair narwhal
#

Alright

#

so as you said its e^cos(inf) basically?

west dome
#

well, its a little more complicated

#

if the inside of the cosine had the sequence 0, 2pi, 4pi, etc, then the inside diverges to inifinity, but cos of the sequence is constant (1) right?

#

essentially, you need to show that $\pi sqrt{n}$ doesn't converge and that it doesn't start repeating values for which cosine is constant

wraith daggerBOT
#

Migillope

west dome
#

i.e. doesnt look like some $\theta + 2\pi k$

wraith daggerBOT
#

Migillope

west dome
#

well, I suppose it could alternate between values on which cosine has the same output, but it doesn't need to be so complicated. Do you see what I'm getting at?

fair narwhal
#

No

west dome
#

ok, let me draw a pic

fair narwhal
#

because for me it looks like an alternating series

west dome
#

the cosine alternates, the e does not

fair narwhal
#

oh yea true e is always positive

#

ok i see now

west dome
#

you can probably use alternating on the cosine, but its complicated

#

anyway, cosine goes around this cirlce

#

if the argument of cosine (here, $\pi \sqrt{n}) keeps landing here, what happens?

#

that should be pi/4

fair narwhal
#

i dont understand, how should i know what cosine(pi*sqrt(n)) is

west dome
#

you mean cosine(pi*sqrt(n))?

fair narwhal
#

yea sorry

#

i just know that cos(...) is either + or -

west dome
#

Okay lets work from there, why is it either + or -

fair narwhal
#

because cosine is either + or -

#

at least for pi*sqrt(n)

#

for lets say 2* n *pi its always 1 like you said

west dome
#

right! So the point is that

  1. pi*sqrt(n) does not converge
    and
  2. It doesnt start repeating values on which cosine is constant. We can show this by nothing that if you take the subsequence where n = 1, 4, 9, 16, etc, our sequence becomes $(\pi \sqrt{1}, \pi \sqrt{4}, ...)$ which is $(\pi, 2 \pi, 3 \pi, ...)$
wraith daggerBOT
#

Migillope

west dome
#

make sense so far?

fair narwhal
#

yep

west dome
#

okay, now what is cosine of that last sequence

#

i.e., $(\cos(\pi), \cos(2\pi), ...)$

wraith daggerBOT
#

Migillope

fair narwhal
#

-1, 1, -1, 1

west dome
#

right!

#

since we we have a divergent subsequence, the entire sequence is divergent

#

this shows that $cos(\pi \sqrt{n})$ is divergent.

wraith daggerBOT
#

Migillope

west dome
#

then, of course, $e^{cos(\pi \sqrt{n})}$ is divergent

#

by any number of tests

wraith daggerBOT
#

Migillope

west dome
#

@fair narwhal would you like to go over it one more time or do you get it?

fair narwhal
#

no it does make sense

#

but what are the accumulation points then?

west dome
#

does it ask for that?

fair narwhal
#

sadly

west dome
#

uh, well. The sequence diverges.

fair narwhal
#

wait but cos can only be -1 or 1 as their maximum points

#

nvm idk

west dome
#

accumulation points are the limit of the sequence

drifting matrix
#

(b) has infinitely many accumulation points, every point in [-1,1] is an accumulaton point.

#

but this is pretty difficult to proof rigourously I think.

west dome
drifting matrix
#

no

#

accumulation point just means that it touches any neighborhood infinitely often.

#

As the picture says in german

west dome
#

ah, I was looking at the wrong defintion

drifting matrix
#

"if NO limit point exists, figure out if there is an accumulation point"

fair narwhal
#

can you explain that for dummies

drifting matrix
#

Luckily it does not ask for a specific accumulation point

#

so there is a simple argument that there exists one without finding one concretely.

drifting matrix
fair narwhal
#

how i can see if theres an accumulation point

#

we always did it with lim sup, lim inf i think

drifting matrix
#

First is it clear to you what an accumulation point is?

fair narwhal
#

ummm

drifting matrix
#

Maybe we should talk about this first?

fair narwhal
#

a point where many (infinite?) points touch it

drifting matrix
#

yes intuitively

#

there are at least 2 formal definitions:

#
  • there exists a subsequence which converges to this point
#
  • a point x such that for any epsilon > 0 and arbitrary N, there exists n >= N such that |x_n - x| <= epsilon
fair narwhal
#

the what now????

drifting matrix
#

it's kind of the same thing

fair narwhal
#

yea i stopped trying to figure out the epsilon thing a long time ago

west dome
#

that's not great

drifting matrix
#

:/

#

indeed

fair narwhal
#

maybe i'll understand it next semester

drifting matrix
#

For our intents and purposes we can work with the first definition, which hides the epsilon inside the convergence.

fair narwhal
#

ok great

drifting matrix
#

so we basically discard any sequence elements we don't like to get a convergent sequence

#

let me give a simple example

fair narwhal
#

so if the sequence diverges all the subsequences diverge too

drifting matrix
#

x_n = (-1)^n

#

this is not convergent as you can see (maybe draw a picture, it goes between -1 and 1 all the time infinitely often)

fair narwhal
#

ok right

#

and the accumulation points are -1 and 1

drifting matrix
#

yep

fair narwhal
#

makes sense

drifting matrix
#

they are touched infinitely often

#

using subsequences x_2n and x_2n+1

#

the even and odd sequence entries

#

okay

#

So I think for our specific example we just need to know one important property of sequences

#

which is often called compactness or Bolzano Weierstraß property

#

it says that if a sequence is bounded then there exists at least 1 accumulation point

#

The intuitiv idea is that the elements must be "somewhere" and since the sequence is bounded they can't escape

#

so somewhere there gotta be a lotta elements

fair narwhal
#

thats true

drifting matrix
#

Given this property you get that there is an accumulation point

#

without doing any real calculation

fair narwhal
#

wait thats so OP

drifting matrix
#

🙂

fair narwhal
#

but the series is divergent

#

or doesnt that matter?

drifting matrix
#

if it is divergent to infinity then you cannot apply it

#

since it's not bounded

#

actually if it divergent to infinity there will be no (real) accumulation point

#

same with minus ininifty of course

fair narwhal
#

Ok

#

but in this case it works because

#

cos is bound to -1 and 1?

drifting matrix
#

yes

#

and exp increasing

#

so

#

1/e = exp(-1) <= exp(cos(...)) <= exp(1) = e

fair narwhal
#

Ok

#

but those are not the 2 accumulation points

#

we dont know them right?

drifting matrix
#

yeah as I said I am pretty sure every number between 1/e and e

#

is actually an accumulation point

fair narwhal
#

that would make sense yea

drifting matrix
#

but luckily we don't need to proof that

fair narwhal
#

alright i understood it now

drifting matrix
#

yay 🙂

fair narwhal
#

thanks to the both of you

drifting matrix
#

You are welcome

fair narwhal
#

.close

cedar kilnBOT
#
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west dome
west dome
#

.close

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odd wharf
#

how do I find the anti-derivative of x / (x^2 + 4)?

odd wharf
#

none of the rules on my trig sheet apply to this

glad kestrel
#

looks like a u sub

odd wharf
#

Symbolab said that

#

But I don't understand how that works with the problem

glad kestrel
#

you have the derivative of your denominator as the numerator, off by a constant

#

normally calls for a u sub

odd wharf
#

uh

#

ok so int [ x / (x^2 + 4) ] dx -> u = x^2 + 4 -> du = 2x dx -> int [x/u] 2x dx

glad kestrel
#

it's a search and replace type thing

#

$\frac{\dd{u}}{2}=x\dd{x}$, agree?

wraith daggerBOT
#

a disappointing son

odd wharf
#

wouldn't it be du/2x? not just over 2?

glad kestrel
#

well i can just divide by the 2 here

#

still the same thing, just keeping x on the other side

#

makes it a bit easier to see

odd wharf
#

wait huh?

#

oh

#

x/u * du/2x

#

x's cancel out

glad kestrel
#

they do, yes

odd wharf
#

1/u * du/2

#

ok so

graceful karma
#

u sub makes me shudder

odd wharf
#

1/u * du/2 = 1/2 * int[ 1/u du]

glad kestrel
#

looks good i think

graceful karma
#

Calculus class leaves scars

glad kestrel
#

don't diss the u sub :(

odd wharf
#

which is 1/2 * ln |u| according to the anti-deriv sheet

glad kestrel
#

indeed it is

graceful karma
#

Calculus students when the integral isnt set up perfectly for u sub to work

glad kestrel
gaunt hamlet
#

Engineers when they're not given a sheet with the answer to every integral in existence:

graceful karma
#

Just tape one to the wall

cedar kilnBOT
#

@odd wharf Has your question been resolved?

cedar kilnBOT
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zenith python
#

.close

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vale jackal
#

Hilf mir

cedar kilnBOT
frigid canyon
#

in which question?

cedar kilnBOT
#

@vale jackal Has your question been resolved?

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dreamy plume
#

what are the possible values of k expressed as an inequality if y=-x+3 and y=k(x+1)-1 intersect in the first quadrant

remote parcel
#

well

#

first lets graph it in desmos

#

we know

#

that we can find a range for k

#

by finding the value of k that would result the second equation passing through (3, 0)

#

and by finding the value of k that would result the second equation passing through (0, 3)

#

the rest is just algebra

dreamy plume
#

ok thanks

cedar kilnBOT
#

@dreamy plume Has your question been resolved?

#
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warm vector
#

How would I do this question?

cedar kilnBOT
livid hound
#

properties of the discriminant

warm vector
#

But There’s an extra variable that’s confusing me

livid hound
#

consider b+1 to be the constant component of the quadratic

remote parcel
#

as long as the D is greater than zero or equal to zero

#

b^2-4ac has to be greater than or equal to zero

#

now

#

the b doesn't actually mean the b in the equation

#

it means the b as in ax^2+bx+c=0

warm vector
#

Ok

#

I don’t really get it

livid hound
#

which part don't you get

remote parcel
#

1-4(2)(b+1)>0 or =0

warm vector
livid hound
#

properties of the discriminant

#

do you know what the discriminant of a quadratic is

warm vector
#

B squared -4ac

livid hound
#

do you know what the variables in that expression represent?

warm vector
#

Yes

livid hound
#

can you apply that to the quadratic you're given

warm vector
#

Except in this question I don’t get what’s the variable C representing

livid hound
#

do you know what the variables in that expression represent?
yes
Except in this question I don’t get what’s the variable C representing...
that's a bit contradictory

#

c represents the part independent of x

livid hound
#

which people refer to as the constant

#

consider b+1 to be the constant component of the quadratic

warm vector
#

Oh ok I get it

#

Ok I got it thanks

#

I have another question

#

How would I do this question?

livid hound
#

same idea

#

properties of the discriminant

warm vector
#

So what would the value of B be?

#

1 I think

livid hound
#

yes

warm vector
#

Ok

#

And the value of A would be k+2 I meant?

livid hound
#

yes

warm vector
#

Ok right

#

Thanks

#

For your help

#

.close

cedar kilnBOT
#
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cedar kilnBOT
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dense dawn
cedar kilnBOT
dense dawn
#

Hi! I have a question on this problem; I don't have any work available at the moment as I don't understand what the question is asking. Is the first step to set the equations equal to each other and then solve from there? But even then, I wouldn't know what to solve for.
(I'm new to the server, please let me know if I'm doing something wrong)
I'm going to sleep soon so it might take me a while to respond after some time

cedar kilnBOT
#

@dense dawn Has your question been resolved?

dire geode
#

damn teacher why you giving proofs to 8th graders

dire geode
wraith daggerBOT
#

riemann

dense dawn
#

So basically turn standard form into vertex form?

dire geode
#

at the end of the algebra, you'll be able to write $h$ as something with $a, b, c$ and same for $k$

wraith daggerBOT
#

riemann

dire geode
#

correct

dense dawn
#

Ohh

west dome
#

8th grade teacher assigned this?? sheesh.

dense dawn
#

Nah, 9th grade lmao

west dome
#

okay still

dire geode
#

oh then that's completely fine /s

west dome
#

my undergrads cant do this

dire geode
#

lol

#

Zizi's in honors algebra for sure

#

80% of her high school grads go to Harvard/MIT/Princeton/Stanford

dense dawn
#

PFFF

#

Okay so I got $a[(x+(b/2a))^2-(b^2/4a^2]+c$

wraith daggerBOT
#

Zizi_Sunee

dense dawn
#

$a$ is the same, $h=b/2a$, and $k=(-b^2/4a)+c$ i think

wraith daggerBOT
#

Zizi_Sunee

dire geode
#

that's my way of telling you that you're almost there

dense dawn
#

hmm ok so now i have to find a, b, and c right

dire geode
#

i mean plug in your values of $h$ and $k$ into the vertex form

wraith daggerBOT
#

riemann

dire geode
#

$a \left(x - \frac{b}{2a}\right)^2 + \frac{-b^2}{4a} + c$

wraith daggerBOT
#

riemann

dire geode
#

does that equal the standard form?

dense dawn
#

no? (guessing at this point)

dire geode
#

gotta do the work zizi

#

you're a future princeton student, gotta work as hard as them

dense dawn
dire geode
#

those are your values, yes

dense dawn
#

i don't really understand what i'm supposed to do next, though 🤔

dire geode
wraith daggerBOT
#

riemann

dire geode
#

you can think of it as a "verify" step

dense dawn
#

hmm alright

#

yes!

dire geode
wraith daggerBOT
#

riemann

dense dawn
#

yes

dire geode
dire geode
dense dawn
#

oh yeah wait gimme a sec

#

yeah ok i see it now

#

so $a(x-(b/2a)^2)+(b^2/4a)+c$

wraith daggerBOT
#

Zizi_Sunee

dense dawn
#

whoops i meant $a(x-(b/2a))^2+(b^2/4a)+c$

wraith daggerBOT
#

Zizi_Sunee

dense dawn
#

Oof I gtg sleep

dire geode
#

so close!

#

it's the other term

#

$(x+h) = (x - (-h))$ my last hint

wraith daggerBOT
#

riemann

cedar kilnBOT
#

@dense dawn Has your question been resolved?

cedar kilnBOT
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pliant sandal
cedar kilnBOT
pliant sandal
#

.close

cedar kilnBOT
#
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cedar kilnBOT
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spiral onyx
#

The number of solutions of the equation [2x]-[x+1]=2x (where [ ] is the greatest integer function) is?

harsh dove
#

@spiral onyx You know what greatest integer function is?

spiral onyx
#

yes but im kind of confused about the question

#

like i haven't solved such questions on GIF before

harsh dove
#

Can you find at least one solution? @spiral onyx

spiral onyx
#

umm

#

im not sure

harsh dove
#

That would be my first attempt on doing such guestions.

#

For example 0 is the solution or not?

spiral onyx
#

how is 0 a solution....

harsh dove
#

I'm asking you, is 0 a solution or not?

spiral onyx
#

i don't think so

#

nvm it's okay I'll ask my teacher tomorrow

#

.close

cedar kilnBOT
#
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indigo plaza
cedar kilnBOT
indigo plaza
#

it is in the 10th power

#

how can i find a pattern to calculate this

dense wing
#

check if it's diagonalizable

indigo plaza
#

oh i see

cedar kilnBOT
#

@indigo plaza Has your question been resolved?

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haughty gyro
#

just a quick question, would i need to use u-substitution to integrate this?

haughty gyro
#

im pretty sure its basic rule but idk

fallen heath
#

nuu, it's standard integration

azure crater
#

hmm, i think it would be easier if you multiply 2 with x^3 and -x and then you just need to integrate each term apart

iron berry
#

you just multiply it out, split the integral and just integrate the polynomials

#

actually you dont even need to multiply it out since integrals are linear

haughty gyro
#

ty

#

do i need to split them or is that just to simplify

#

its in the context of this problem

graceful karma
#

Or just extract the constant outside the integral

haughty gyro
#

ah

#

Seems like i messed up somewhere

graceful karma
#

Wouldnt they want you to evaluate it

#

I guess that doesnt evaluate to the right answer

#

Also whyd you split the integral?

haughty gyro
#

it doesnt care if i evaluate so i usually dont

#

just so theres less margin for error

#

and i split it because of the two different areas

graceful karma
#

Id just do it in one integral

#

Since its all one function

gaunt hamlet
#

You have to deal with that negative area, no?

haughty gyro
#

yeah so i set it up as

#

well idk how to use the bot to write it so ill take another picture

graceful karma
#

,rotate ccw

wraith daggerBOT
haughty gyro
#

srry

#

its the bottom one

#

the integration from -1 to 0 + the integration from 0 to 1

graceful karma
#

Why not just fundamental theorem it

#

F(1)-F(-1)

#

Rather than F(1)-F(0)+F(0)-F(-1)

haughty gyro
#

ohhh

graceful karma
#

The F(0) cancels out anyways

haughty gyro
#

alright ill try that

#

i still got it wrong

#

i got ((1/2) - 1) + ((-1/2) + 1)

gaunt hamlet
#

You have to do the integral from -1 to 0 MINUS the integral from 0 to 1 to account for the negative area, I think

haughty gyro
#

didnt work either

gaunt hamlet
#

What did you input?

graceful karma
#

The integral would already be negative

#

F(-1) for example would be -0.5

gaunt hamlet
#

But we don't want the signed area, no?

haughty gyro
#

i just tried subtracting instead of adding

gaunt hamlet
#

So we gotta account for that?

graceful karma
#

Oh is that what they want?

gaunt hamlet
#

I'm assuming based on the wording

graceful karma
#

Usually Id think that meant the negative area too

#

Irdk though

haughty gyro
#

i figured it out

#

i missed a negative sign somewhere facepalmAether

#

ty

graceful karma
#

Wait was it total area?

#

Or like positive negative

#

Now Im curious

haughty gyro
#

it was total area

#

so the first part on the left was 1

#

and the second part was 1 as well

#

answer was 2

#

i honestly thought they would cancel out but here we are

gaunt hamlet
#

They would cancel out if it was signed area

cedar kilnBOT
#

@haughty gyro Has your question been resolved?

cedar kilnBOT
#
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cedar kilnBOT
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umbral marsh
#

how do I solve

cedar kilnBOT
umbral marsh
#

Im making a function that returns a and b

#

x1, y2 can anywhere, behind it, above it, below it, in front of it, etc.

crimson sedge
#

Oh. Uh one sec i think i can help with this one

umbral marsh
#

first I need the angle I'm pretty sure

#

ok now it looks better

crimson sedge
#

you could go about it using trig

umbral marsh
#

I just need a and b's values

umbral marsh
crimson sedge
#

but i think you can also go about it using scaling i think, that might be easier, do you want that way?

umbral marsh
#

just any solution I can understand / implement in programming

crimson sedge
#

do you understand basic trig functions?

umbral marsh
#

yes

#

sin, cos, tan, asin, acos, atan and the cs I cant memorize their names

crimson sedge
#

oh that does noy have have a back ground

#

hold on wait, i really think this will be easier with just scaling

umbral marsh
crimson sedge
#

give me a moment to work out a gaood way to explane

#

so if we have a rectangle and we cut the side lengths in half the resulting red rectangle is 1/4 the size in area

#

and if we make the length of the rectangle 1/3 then the area is 1/9

#

what im trying to show here is that there is a relationship between scaling so and the area

umbral marsh
#

k

#

i think I get what ur saying

#

what's the scale in this

#

I can figure out longer sides

#

and the long line in the middle

#

I know the small line in the middle in the small rectangle is 10

crimson sedge
#

so if we make the rectangle
1/2 length the area ia 1/4
1/3 length the area is 1/9

so if se scale the rectangle to
1/x length the area is 1/x^2

#

ON WAIT

#

the length of that line is 10? no the area of the small rectangle?

umbral marsh
#

length of that line is 10

#

sorry

#

I didn't clarify

crimson sedge
#

lmao, i was assuming the area of that square is 10

#

this is a much easer question now lol

#

when you uniformly scale a triangle the ratio between the side length stays the same

umbral marsh
#

ok one sec

crimson sedge
#

let me know when ever you ready to contine

umbral marsh
#

LOOOOOLL

#

THIS QUESTION IS SO EASY

#

WHY DID I NOT THINK OF SCALING

#

TYSM

#

.close

cedar kilnBOT
#
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keen thunder
#

Is 24° correct?

cedar kilnBOT
keen thunder
#

If not what's the correct answer?