#help-49

1 messages · Page 65 of 1

twilit field
#

oh shit

dreamy lichen
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how do you know that?

subtle blaze
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shouldn't you have a fullstop after the set

twilit field
#

sorry

subtle blaze
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then say ||suppose T is non-empty||

twilit field
#

oh, right

dreamy lichen
twilit field
#

I've made that change

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on my end

dreamy lichen
#

okay good, next thing is not necessary but i think its good to include it anyway. You can mention that you are using WOT. Like "By WOT, T has a smallest element, so let a be the smallest element of T."

twilit field
#

got it

subtle blaze
#

is that what WOT says

dreamy lichen
#

And you should expand on the => (a-1) ∈ S

dreamy lichen
#

and T is non-empty subset of Z+

fleet moss
#

i am still confused what we are even trying to prove here. if you want to say that this construction S is equivalent to Z+, we first need some definition of Z+ to compare it to.

this contradicts the definition of T
why does it contradict it?

twilit field
#

T is defined to consist of those elements that aren't in $S$, but I've shown that that's not true

grand pondBOT
#

ƒ(Why am. I here)=I don't Know

dreamy lichen
#

basically, a is both element of S and T

subtle blaze
#

well i've just used Z+ as a scapegoat so he stops saying that T contains everything not in S

dreamy lichen
#

so thats how the contradiction arises

dreamy lichen
#

also you can just generally expand on that implication and explain it in more detail

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like a-1 isnt in T, lest it would be the smallest element

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hence either a-1 isnt in Z+, or it's in S

fleet moss
dreamy lichen
fleet moss
twilit field
#

Sorry for making you spend so much of your time helping me on a simple proof .

dreamy lichen
#

hence a-1 is either not in Z+ or it's in S

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(by definition of T)

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the only way a-1 wouldnt be in Z+ is a-1 = 0 and a = 1, but 1 is in S, so 1 can't be in T and so it cant be minimal element of T

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so a-1 must be in S

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and then a must be in S by definition of S

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this all of that should be at least partially included in the proof btw

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maybe not in that much detail, but it should

subtle blaze
#

when you do it for the first time you should write it in that detail and more

dreamy lichen
#

that's true too

subtle blaze
#

until you know you can do it, you should write it in that detail

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if you're just going to handwave the parts you dont know then you might as well not be doing proofs

twilit field
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got it, sorry

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should I write it afresh ?

fleet moss
grand pondBOT
#

esca (@ with reply)

dreamy lichen
#

i used disjunctive syllogism or smth like that

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idk how its called

#

a-1 is either not in Z+ or it's in S

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I showed that a-1 is in Z+

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hence its in S

fleet moss
dreamy lichen
subtle blaze
#

small a

dreamy lichen
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yeah

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and so the only way for a-1 to not be in Z+ is a-1 = 0

fleet moss
subtle blaze
#

you dont know

subtle blaze
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until that

dreamy lichen
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then i made argument that a cant be 1

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since 1 is in S

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and so 1 cant be in T

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and a fortiori it cant be minimal element of T

fleet moss
grand pondBOT
#

esca (@ with reply)

subtle blaze
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no

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we're trying to prove that S = Z+

dreamy lichen
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Well I assume there are some properties about integers that are already established

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and this one looks trivial enough

fleet moss
twilit field
subtle blaze
dreamy lichen
#

not really

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we are proving all positive integers is subset of S

subtle blaze
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sorry yeah

dreamy lichen
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we just have some properties that we can use

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i assume trivial arithmetical properties

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which is what i used

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and WOT

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which is taken as axiom

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in the book

fleet moss
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what is an integer i guess should be my question

dreamy lichen
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why care about that

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we have some properties

subtle blaze
dreamy lichen
#

and we dont care whether 2 = {∅, {∅}} or 2 = {{∅}}

subtle blaze
#

we're not looking to prove absolutely everything from scratch

fleet moss
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so we take for granted that if a is a positive integer, then a+1 is also a positive integer

twilit field
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yes, I guess

fleet moss
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we also take for granted that 1 is a positive integer

dreamy lichen
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yes

fleet moss
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and we take for granted that a +b where b isnt an integer is not an integer

dreamy lichen
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no?

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1/2 + 1/2 = 1 which is an integer

fleet moss
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a is still an integrt

dreamy lichen
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okay, then yes i guess?

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but this is already much less trivial than what i used

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look, we just take some basic arithmetical properties for granted, and then we use WOT to derive new a base for new "proof technique", which is induction

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I think we can assume that if a is in Z+, then a-1 is in Z+ or a-1 = 0

#

its equivalent to this btw:
if a is non-negative integer, then a+1 is positive integer

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and that's just trivial

twilit field
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Uh, so is anything still wrong with my proof?

dreamy lichen
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What's the current version of it?

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this?

twilit field
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this

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yes

dreamy lichen
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I dont like this part

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you should explain in more detail why (a-1) is in S

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start by (a-1) not being in T, by minimality of a

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then use definition of T, exclude the possibility that (a-1) is not an integer and conclude that (a-1) is in S

twilit field
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sorry, this is it

fleet moss
dreamy lichen
dreamy lichen
twilit field
#

Ok

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other than that?

dreamy lichen
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you repeat the same thing twice actually

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delete the first one and it will be fine

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and seperate your sentences

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suppose T is non-empty let a be the smallest element of T (a-1) in S which by definition...

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this should be 3 sentences

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other than that, you should expand the proof there

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explain why (a-1) should be in S

twilit field
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ok, so that's basically as if $k\in S$ as is $k+1$

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right

grand pondBOT
#

ƒ(Why am. I here)=I don't Know

dreamy lichen
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What do you mean?

twilit field
dreamy lichen
#

Currently, you have that a is the smallest element of T. Use that and proof that (a-1) is in S

twilit field
#

by definition...

dreamy lichen
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But what you need to explain is that if a is smalelst element of T, then (a-1) is in S

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this is the big jump in your proof

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it's correct, but the jump was too long

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you skipped like solid 2 lines of proof

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and not exactly trivial one

fleet moss
# dreamy lichen no it's not?

youre saying the only thing we know about Z+ is it has certain properties. and we're constructing S as some set that has exactly the same properties. then i dont understand whats left to prove

dreamy lichen
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we have some set S that has the properties that it contains 1 and is closed under successor

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and we are proving that it contains all positive integers

fleet moss
twilit field
#

I'm really confused now, sorry

dreamy lichen
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we dont always need to use the same construction with same properties

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can we move this to DMs esca?

fleet moss
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sure

dreamy lichen
fleet moss
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sorry physics rocks

twilit field
#

ok, so if $a-1$ is in S , as is $a$

grand pondBOT
#

ƒ(Why am. I here)=I don't Know

dreamy lichen
# twilit field here

I suppose this is supposed to be

"Let a be the smallest element of T. Then (a-1) is in S.

#

Is that right?

twilit field
#

yes

dreamy lichen
#

okay, that's not so trivial conclusion though

twilit field
dreamy lichen
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but it doesnt show that if a is the smallest element of T, then (a-1) is in S

twilit field
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by construction if an element isn't in T , it has to be in S

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assuming it's an integer

dreamy lichen
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exactly

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and that's not so trivial

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and you stuffed all of that in one single step

twilit field
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ooh, ok

dreamy lichen
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while it should be mentioned in the proof

twilit field
#

got it

dreamy lichen
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you need to say that (a-1) is not in T, which follows from minimality of a

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then you need to establish that (a-1) is a positive integer

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and only then you can conclude that its in S

twilit field
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oh, I have to do that too?

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ok

dreamy lichen
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and partially

twilit field
dreamy lichen
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meaning somewhere in between those 2 things

twilit field
#

ok

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and thanks so much for all the help

dreamy lichen
#

np

twilit field
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one minute

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I'm lost

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I think I'll just try this again later, sorry

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is that fine?

dreamy lichen
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yeah

twilit field
#

ok

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sorry

dreamy lichen
#

np

twilit field
#

.close

midnight plankBOT
#
Channel closed

Closed by @twilit field

Use .reopen if this was a mistake.

dreamy lichen
#

you can finish it whenever you want, the progress is already quite good

#

the proof outline is correct

twilit field
#

👍

last slate
#

Can any1 help me with this

midnight plankBOT
#
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versed ridge
#

for part b, i had $\int_{0}^{1} \int_{x^2-x+1}^{1} \int_{2-x-y}^{1-x^2} (x) dz dy dx$. I am wondering why this is wrong.

grand pondBOT
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Secret

sweet wing
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do you have the drawing of the regions?

versed ridge
#

no but i could make one rq

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here's what the region looks like

midnight plankBOT
#

@versed ridge Has your question been resolved?

versed ridge
#

<@&286206848099549185>

midnight plankBOT
#

@versed ridge Has your question been resolved?

midnight plankBOT
#

@versed ridge Has your question been resolved?

pearl idol
#

That geogebra drawing looks terrifying lol

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Lemme take a look at the problem

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where did you get the bounds for y from? for example, couldn't y be 2? (if x=z=0)

midnight plankBOT
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@versed ridge Has your question been resolved?

versed ridge
#

and that gave me $2-x-y=1-x^2$

grand pondBOT
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Secret

versed ridge
#

which meant $y=1-x+x^2$

grand pondBOT
#

Secret

pearl idol
#

That makes sense for the lower bound, but what about the upper bound of 1?

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Where did you get that from

versed ridge
#

i think i figured that if $z=1-x^2$ and we are bounded by the plane $z = 0$, $-1 \leq x \leq 1$ but we only really consider $0 \leq x \leq 1$. Now, looking at the second surface, we have $z=2-x-y$. If $z \geq 0$, then $x+y<=2$ and i think i plugged in $x=1$ into this inequality.

grand pondBOT
#

Secret

versed ridge
#

tho ig i should plug in x=0 if i were to find the upper bound for y

pearl idol
#

yeah

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I think your upper bound for y should probably depend on x

versed ridge
#

oh so 2-x

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ok yea that makes sense

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thank you!

#

.close

midnight plankBOT
#
Channel closed

Closed by @versed ridge

Use .reopen if this was a mistake.

pearl idol
#

Wait does 2-x actually work lol I haven't checked

versed ridge
#

imma try it out and lyk

pearl idol
#

kk good luck!

versed ridge
#

ok this is the revised integral and it gives me the wrong answer. Do you see anything else wrong with it?

pearl idol
#

.reopen

midnight plankBOT
#

pearl idol
#

So my question is

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Are you sure you're looking at the right region

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Because the bounds you've set make me think you're looking at this region

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Between the blue parabola and the red plane, and so on

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But it seems like that region would just continue on forever to the right catthink

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Lemme try plotting it

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what I'd assume is you're trying to find this region, under both the plane and the paraboloid?

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In that case, you should have that x+y+z ≤ 2 and x^2 + z ≤ 1

versed ridge
#

ye i think the one that is bounded should be the region

pearl idol
#

Before, you were assuming, I think, that x+y+z ≥ 2 in order to find lower bounds for y and z

versed ridge
#

ye

pearl idol
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The inequality should be the other way around though

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So that we can have a bounded region

versed ridge
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wait so if the region is under the plane

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then would it be less than or equal to the plane

pearl idol
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yea

versed ridge
#

i see

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ok i'll try this out

pearl idol
#

So you've got
0 ≤ x
0 ≤ y
0 ≤ z
x + y + z ≤ 2
x^2 + z ≤ 1

versed ridge
#

ye

pearl idol
#

I would prob do the outside integral 0 ≤ z ≤ 1

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Although, not really sure

versed ridge
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wait wouldn't it be z between 0 and 2

pearl idol
#

No bc x^2 + z ≤ 1

versed ridge
#

oh yea you take the lesser bound

pearl idol
#

Yeah this is tricky

versed ridge
#

we can do y as innermost

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and then go from 0 and 2-x-z

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and x goes from 0 to 2-z

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does that make sense?

pearl idol
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Then you're not really using x^2 + z ≤ 1, are you

versed ridge
#

ye ig i have to take cases for x

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i have two possible inequalities $0 \leq x \leq (2-z)$ and $0 \leq x \leq \sqrt{1-z}$

grand pondBOT
#

Secret

midnight plankBOT
#

@versed ridge Has your question been resolved?

pearl idol
#

0 ≤ x ≤ 1
0 ≤ z ≤ 1 - x^2
0 ≤ y ≤ 2-x-z

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Do these bounds work? @versed ridge

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They feel like they should

versed ridge
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let me check

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yep that's right

pearl idol
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Awesome

versed ridge
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how did you get there?

pearl idol
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y always goes from 0 to the plane

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So I figured that one should go on the inside of the integral

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Once we've got that covered, that ensures we're always under the plane

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And then we just need to deal with the x and z being inside the parabola

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You can either do this like
0 ≤ x ≤ 1
0 ≤ z ≤ 1-x^2
or I think
0 ≤ z ≤ 1
0 ≤ x ≤ sqrt(1-z)
should work fine too

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And then that solves it

versed ridge
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ah i see

pearl idol
#

So like, even though the region of integration is somewhat complicated, if you project it onto the x-z plane, it's actually really simple

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(just the area under a parabola)

versed ridge
#

ig you should be able to do it with x and z as innermost too, right? it would just involve taking two different cases

pearl idol
#

I think it would be a lot more complicated than this way

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You can try it

versed ridge
#

i see either way, thanks a lot, man!

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appreciate it

pearl idol
#

you're welcome!

versed ridge
#

.close

midnight plankBOT
#
Channel closed

Closed by @versed ridge

Use .reopen if this was a mistake.

midnight plankBOT
#
Available help channel!

Send your question here to claim the channel.

Remember:
Ask your math question in a clear, concise manner.
Show your work, and if possible, explain where you are stuck.
After 15 minutes, feel free to ping <@&286206848099549185>.
• Type the command .close to free the channel when you're done.
• Be polite and have a nice day!

Read #❓how-to-get-help for further information on how to ask a good question, and about conduct in the question channels.

tidal turret
#

let $f(x) = x^2 \ln x$ $\\1)$ how many solutions does the equation $f(x) = \frac{-1}{6}$ have $\\$2) for which values of $k$ the equation $f(x) = k$ has exactly one solution

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

first one is doable

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,, \frac{-1}{6} = x^2 \ln x

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

well

frank wolf
#

Try sketching the graph

twilit field
#

I don't think you actually need to solve it

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try finding the number of turning points and the concavity of the graph

tidal turret
#

how to sketch the x^ 2 lnx doe

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ok so lets calculate the critical points and the points of inflection, no?

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wdym concavity

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how do I find concavity and convexity of this

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?

twilit field
#

I don't meanu numerically

tidal turret
#

,, x^2 \ln x \ \frac{d}{dx} \left(x^2 \ln x\right)

twilit field
#

I mean is it concave up or concave down

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or neitger

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

neitger?

twilit field
#

,w graph x^2ln(x)

twilit field
#

notice it;s concave up

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on (0,1)

tidal turret
#

sure

#

what about the derivatives

twilit field
#

what's the derivative of x^2ln(x)

tidal turret
#

,align x^2 \ln x = \ \frac{d}{dx} \left(x^2 \ln x\right) &= \frac{d}{dx} \left[x^2\right]\ln(x) + (x^2)\frac{d}{dx}\left[\ln(x)\right] \ &= 2x \ln(x) + \frac{x^2}{x} \ &= 2x \ln(x) + x

twilit field
#

pretty sure that's wrong

#

what's the derivative of ln(x)

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yes

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this is right

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now notice it's 0 at a point between 0 and 1

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

ln(x) in here x > 0 strict

last slate
#

ye so e^(-1/2) is the exttema

tidal turret
#

wdym

last slate
#

factor x out

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in the derivative

tidal turret
#

,, 0 = 2x \ln(x) + x \ 0 = x(2 \ln(x) + 1)

last slate
#

ye

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

so either x = 0 or
2ln(x) + 1 = 0

last slate
#

no

#

x=0 isnt possible

tidal turret
#

okay

#

why doe

last slate
#

cuz f(0) doesnt exist

tidal turret
#

okay

#

sure

last slate
#

ye

tidal turret
#

then $2 \ln(x) + 1 = 0$

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

thats why e^-1/2

last slate
#

ye

#

u can ignore x in the derivative

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so the function is ez to plot

tidal turret
#

,w solve 2\ln(x) + 1 = 0

grand pondBOT
tidal turret
#

e^-1/2

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now what

last slate
#

can you draw 2lnx + 1

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if you can ur gonna realize that f(x) is gonna look like a quadratic

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with vertex at x = e^(-1/2)

tidal turret
#

how do I draw it doe

last slate
#

you dont know how lnx looks like?

tidal turret
#

,w plot ln(x)

grand pondBOT
last slate
#

ye

tidal turret
#

so what?

#

can you draw 2lnx + 1
how

last slate
#

it looks pretty much the same

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you dont have to be precise when using derivatives to check where the function is increasing/decreasing

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thats why i ignored x in the first place

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cuz x>0

tidal turret
#

sure

#

now what

last slate
#

u did it?

tidal turret
#

no I still cannot

#

can you draw 2lnx + 1
how

last slate
#

if you cant do it you can check where 2lnx+1 is positive n negativd

tidal turret
#

how did you do it

#

I want to know the deepfriedpack way of doing stuff

last slate
#

its simple

last slate
#

so this

tidal turret
#

here root is 1?

last slate
#

ye but its now 2lnx+1

tidal turret
#

I mean in lnx

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damn

#

got it got it

last slate
#

yeah but were doing 2lnx+1

tidal turret
#

sure

#

now what

#

,calc e^(-1/2)

grand pondBOT
#

Result:

0.60653065971263
last slate
#

u now know where the function is increasing or decreasif

tidal turret
#

exactly

#

(e^-1/2, +infty) increasing

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mmm

#

still confused

#

need more hints

last slate
#

gimme a sec

tidal turret
#

sure, I will scroll tiktok real quick

last slate
tidal turret
#

unsure how to answer 1 and 2 from my original question

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I am getting confused all over again

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I need more handholding

last slate
tidal turret
#

fx?

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x^2 ln(x)

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how would I

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do that

last slate
#

so

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something like this

tidal turret
#

(0, e^-1/2) is decreasing

last slate
tidal turret
#

(e^-1/2, infty) increasing

last slate
#

u dont nesd that much detail

tidal turret
last slate
#

f(x)

last slate
tidal turret
#

how

#

need more hints

last slate
#

i just used the definition of decreasing n increasing

tidal turret
#

okay

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I get it now

#

now what?

last slate
#

whats f(e^(-1/2))

tidal turret
#

tangent zero

last slate
#

not the derivative

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the actual function

tidal turret
#

,calc e^(-1)*-1/2

grand pondBOT
#

Result:

-0.18393972058572
last slate
#

ye now compare that to -1/6

tidal turret
#

,calc -1/6

grand pondBOT
#

Result:

-0.16666666666667
last slate
#

so -1/6 abit bigger

tidal turret
#

,calc -1/5.5

last slate
#

mb

grand pondBOT
#

Result:

-0.18181818181818
last slate
tidal turret
#

its not 1/6 doe

last slate
#

what

tidal turret
#

,w 1/sqrt(e) = 1/6

last slate
#

-1/6 is abit bigger than f(e^(-1/2))

grand pondBOT
tidal turret
last slate
tidal turret
#

okay

#

now what

last slate
# last slate

so can u tell how many roots there are in f(x) = -1/6

tidal turret
#

1

last slate
#

why

tidal turret
last slate
#

ye how many intersections are there

tidal turret
#

two

last slate
#

yeah

#

try the second one now

tidal turret
#

how?

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,, x^2 \ln x = k

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

this ?

last slate
#

ye

tidal turret
#

how do I do that

last slate
#

k = -1/6 had two roots

tidal turret
#

okay

last slate
#

what should k be so that those 2 have 1 intersection

tidal turret
#

how would I know that

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the intersection has to be at the turning point

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at the vertex of the parabola

last slate
#

possible ks

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actually nvm its a range

tidal turret
#

how do I do that

last slate
#

is there a condition that says k <0

tidal turret
#

yeah definetely

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x^2 is postiive

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,w range of ln(x)

grand pondBOT
tidal turret
#

wait, log can output negative aswell?

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didnt knew that

last slate
#

in the og question

last slate
tidal turret
#

let $f(x) = x^2 \ln x$ $\\1)$ how many solutions does the equation $f(x) = \frac{-1}{6}$ have $\\$2) for which values of $k$ the equation $f(x) = k$ has exactly one solution

#

wdym

grand pondBOT
#

938c2cc0dcc05f2b68c4287040cfcf71

tidal turret
#

I dont get it

#

wdym

#

@robust cloak

midnight plankBOT
#

@tidal turret Has your question been resolved?

tidal turret
#

Can someone help my fatass

last slate
hollow oyster
#

I think you have to use the W Lambert function

#

Like let lnx^2=u

#

Then x^2=e^u

hollow oyster
hollow oyster
upper crag
#

lambert w won't help, it's not in most curriculums

hollow oyster
#

Oh I see

#

But it will give the solution

upper crag
#

some calculus will help, basically 1 solution occurs at the min of f if we think of this k value as a vertical translation

hollow oyster
#

I check in Wolfram also

#

Then what do you suggest mero

hollow oyster
#

I'm not sure if I understand that

upper crag
#

what part

hollow oyster
#

The assumption that k is a vertical translation

upper crag
#

maybe this is convincing

hollow oyster
#

Also how can we be sure there's only 1 solution

upper crag
#

move k so that the minima lies on the x-axis

hollow oyster
#

Ok i see

upper crag
#

also these are increasing functions on the domain [0,infty)

#

ok cool I'll let you work on it now

midnight plankBOT
#

@tidal turret Has your question been resolved?

hollow oyster
#

Mero my dumb brain is not getting what to do T_T

upper crag
#

well ok first off, were you able to solve for the x value of the minima?

hollow oyster
#

1?

upper crag
#

nope

#

f(x)=x^2ln(x)

#

now the minima occurs when f'(x)=0

hollow oyster
#

e^-1/2

upper crag
#

good

#

how did you get that can you show your work

#

that was too quick like you're just plugging this into wolfram alpha 😛

#

that's no fun

#

well regardless once you have that, then plugging e^{-1/2} into f will be the height of the minima

last slate
midnight plankBOT
hollow oyster
last slate
hollow oyster
#

Lol

upper crag
#

eh?

#

2xln(x)+x=0

since x !=0 isn't in our domain, you divide it out

2ln(x)+1 = 0

hollow oyster
last slate
#

,rotate

grand pondBOT
hollow oyster
upper crag
#

oh ok cool

last slate
# grand pond

was this just so that we can see the eye drawing more better 😭

upper crag
last slate
upper crag
#

lmao

upper crag
last slate
last slate
hollow oyster
#

I got e^-1/2 as min

upper crag
#

specifically the x value of the min

#

we want the y value of the min since that's how much we need to translate the graph by so that it's the only point tha ttouches the x-axis

hollow oyster
#

1/2e

upper crag
#

I think it's -1/(2e) unless you mean the translation amount up

hollow oyster
#

Thus k=1/2e

hollow oyster
upper crag
#

then k=-1/(2e)

hollow oyster
#

My bad

upper crag
#

all good

hollow oyster
#

Well how to do the first w/o W Lambert function?

upper crag
#

same sorta calculus stuff

hollow oyster
#

But it is different

#

Man i have to revise my clac

#

Well it is my hw for the week

#

Bye mero

upper crag
#

nite nite

last slate
#

k>=0 also works though

last slate
#

the answer should be a range

hollow oyster
last slate
hollow oyster
#

I'm just not gonna slove this

last slate
hollow oyster
#

So "bye"

hollow oyster
hollow oyster
midnight plankBOT
#
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bleak pier
midnight plankBOT
bleak pier
#

hints please

sage helm
#

what is an "intermediate" value

deep vine
#

over what interval?

#

intermediate value theorem requires an interval

bleak pier
bleak pier
#

and we do then derivative

deep vine
#

but what is the interval

bleak pier
#

as we can see it is not given

sage helm
#

Half the questions you bring are incomplete in some way

#

Do you just need us to confirm that there's nothing to be done?

bleak pier
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bleak pier
midnight plankBOT
bleak pier
#

Hints please

sage helm
#

Do you have expressions for the volume and surface area

#

You need a function for the volume in terms of your favorite conical parameter

midnight plankBOT
#

@bleak pier Has your question been resolved?

bleak pier
#

.cl;soee

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midnight plankBOT
#

@flint heron Has your question been resolved?

flint heron
#

<@&286206848099549185>

midnight plankBOT
#

@flint heron Has your question been resolved?

iron pasture
#

you seem to have solved the problem already.

#

p is congurent to 1 mod 4 only if p-1 is divisible by 4

flint heron
#

Yes, I am not sure why that is. Should I learn about Legendre symbols?

#

This topic wasn't covered in my textbook

iron pasture
#

One sec let me check some stuff

flint heron
#

Thank you!

iron pasture
#

It seems like you need to use Legendre symbols for this. Is this problem from your textbook?

flint heron
#

Not directly from it. I will look this up and try to figure out and come back if I get stuck. Thank you

#

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summer ravine
#

solve the following system of equal for real x,y,z,;
x+y-z = 4
x^2-y^2+z^2 = -4
xyz = 6

summer ravine
#

.close

midnight plankBOT
#
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summer ravine
#

.reopen

midnight plankBOT
#

summer ravine
#

.close

midnight plankBOT
#
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summer ravine
#

nvm

#

.reopen

midnight plankBOT
#

summer ravine
#

help me solve this please

#

x+y-z = 4
x^2-y^2+z^2 = -4
xyz = 6

#

real solutions

#

helpers

#

nvm i got it.

#

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steady trail
#

Guys, but as I'm practicing finding the maxima and minima of $f(x,y)=\arcsin(x^2-y-1)$ I get some derivatives which seem infinite. Is there a way to do it faster, like parameterization?

grand pondBOT
steady trail
#

Domain D:
${(x,y) \in \Bbb R^2 : 0 \leq x^2-y \leq 2}$

grand pondBOT
midnight plankBOT
#

@steady trail Has your question been resolved?

steady trail
#

<@&286206848099549185>

junior flower
steady trail
#

Yes

junior flower
#

so now just find a pair (x,y) that makes arcsin(x^2 - y - 1) = -pi/2 and another pair that makes arcsin(x^2 - y - 1) = pi/2

#

this shows the min and max are -pi/2 and pi/2 respectively

steady trail
#

but what gives us the certainty that it takes on those values

junior flower
#

we can write the pairs explicitly

steady trail
#

for Extreme value theorem ?

junior flower
#

no

steady trail
#

I mean how do we know that the function certainly has a maximum and minimum

junior flower
steady trail
#

Okay

junior flower
#

if you want arcsin(x^2 - y - 1) = -pi/2, you want x^2 - y - 1 = -1

#

pick any x,y that satisfy that equation

steady trail
#

So when x^2-y=0

#

x=0, y=0

junior flower
#

yep that works

steady trail
#

But couldn't there be others now?

junior flower
#

other x,y that make arcsin(x^2 - y - 1) = -pi/2?

steady trail
#

x=1, y=1

junior flower
#

yea but that doesn't matter

steady trail
#

Why?

junior flower
#

we have arcsin(x^2 - y - 1) >= -pi/2 for any x,y, and there exist x,y with arcsin(x^2 - y - 1) = -pi/2

#

that shows the min is -pi/2

#

think about the minimum value of sin

#

it's -1

#

and we have sin(x) = -1 for lots of different x's but that doesn't matter, the min is -1

steady trail
#

Indeed

#

So max

#

x^2-y=2

#

x=-1 , y=-1

junior flower
#

yep that works

steady trail
#

But in fact, seeing the domain, setting x=0 and y=0, solving the equation I find 0≤0

#

And setting x=-1 and y=-1 i find 2≤2

#

Here I find precisely the extremes of the domain

junior flower
#

sure

#

that's expected since arcsin is an increasing function

steady trail
#

Last question: but what does it mean to parameterize this function

junior flower
#

wait no that's not what i meant

steady trail
#

Okay

junior flower
#

nvm i can't explain this well

steady trail
#

Maybe with an example ?

junior flower
#

well i can just give my stupid explanation again. the graph of f consists of points of form (x, y, arcsin(x^2-y-1)) where (x,y) is in D. call this set of points G
paramaterizing f means making some real variable functions a,b,c and a set A where [the points of form (a(t), b(t), c(t)) for t in A] is G

#

so you can get a point on the graph of f by just taking one real number t and looking at the point (a(t), b(t), c(t))

#

this is how i think about it but it's maybe too abstract

#

<@&268886789983436800>

visual tiger
#

<@&268886789983436800>, no advertising especially in help channels

junior flower
#

(spammed in lots of channels)

steady trail
#

Thank you so much

steady trail
#

.close

midnight plankBOT
#
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whole lantern
midnight plankBOT
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sonic dragon
midnight plankBOT
sonic dragon
#

it doesnt work

old storm
#

Why the hell did u close your previous one

lusty cave
#

lol

old storm
sonic dragon
#

can some one just please help me

lusty cave
violet storm
#

maybe the answer form expects the answer fed to it in some specific way, sounds like a pain with this problem. maybe worth trying to factor out the e^ part out of both factors

#

as far as doing it correctly, seems ok to me

old storm
#

Wait

#

Your answer is right

#

Wtf

sonic dragon
#

yea electonic maths things arnt well suited for this i guess

old storm
#

Hmm

#

Remove one pair of brackets

#

From the second part

violet storm
old storm
#

Also the brackets

#

You have a pair of useless brackets

violet storm
#

oh wait you forgot a parenthesis here, this will do it

old storm
#

Or that!

sonic dragon
#

whats a parentisis?

violet storm
#

( )

#

...

sonic dragon
#

oh thanks

old storm
old storm
sonic dragon
old storm
#

Why do you have 5he outer brackets

violet storm
#

sorry i should have known to use UK english given your username and profile pic : )

old storm
#

Remoce them

sonic dragon
old storm
#

Bottom one

sonic dragon
#

ayy it was right

#

nice

#

thank you

violet storm
#

sure...the outer bracket isn't really necessary

#

but i guess doesn't matter

sonic dragon
#

i really dont like simplifying

#

thank you

#

🫡

#

.clsoe

#

.clsoe

#

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thorny glen
#

Anyone have an idea of how solve this integral

midnight plankBOT
old storm
#

Write sin theta as u

rigid mirage
#

u = sinx

#

du = cosx dx

#

so
you do

vocal inlet
#

where do u see x

rigid mirage
violet storm
#

easier to type in discord than theta 😛

rigid mirage
#

:)

vocal inlet
#

damn these mathematicians they see x everywhere

vocal inlet
rigid mirage
#

so

#

so

old storm
rigid mirage
violet storm
grand pondBOT
#

∫oosh (lemonsaurus appreciator)

violet storm
#

just rename the variable :p

old storm
#

There is a hentai joke here somewhere with the x but for the life of me I cant think of it

rigid mirage
#

i mean

#

with doing u = sinx

#

it still won't work thinkies

thorny glen
#

I got it from this integral when x is sin(theta), I’m not sure if this integral is easily solved, if not it’s fine but would like to know the actual solution

violet storm
#

did you use a trig sub?

thorny glen
#

Unless I messed up somewhere in the bounds

#

Yes

old storm
#

You lost me

violet storm
#

you used x = sin t ?

thorny glen
violet storm
#

seems like what you would do

old storm
#

I wonder if you should just polar this

violet storm
#

looks like a pretty standard trig sub integral

vocal inlet
#

okay nevermind

thorny glen
#

The cosine is tripping me up

old storm
#

Does that to all of us sometimes

thorny glen
#

Is there a way to turn cos(sin(x)) into something

vocal inlet
#

like the weeb above said

#

u = sin x

thorny glen
#

That would just reverse the substitution though

#

Btw this isn't a homework problem just made the integral up and try to solve it

violet storm
#

maybe you just stumbled upon a hard integral then

#

kinda easy to do with integrals :p

#

but attempting it with trig sub seems like a good thought at least

violet storm
thorny glen
#

It came from this integral, I only made it this far

#

.close

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#
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last slate
#

HELO

midnight plankBOT
last slate
#

must be 8?

#

wait

#

would it be half the length

#

i forgot about this not going to lie

#

any ideas?

#

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bleak pier
midnight plankBOT
bleak pier
#

hints please

olive matrix
#

just evaluate it

#

order of operations

bleak pier
#

what?

olive matrix
#

BODMAS or whatever

#

start with the exponents

bleak pier
#

it is going to infiity

olive matrix
#

is it? it doesn't look like it to me

fathom tendon
#

i guess theres just a hidden ... at the right

bleak pier
olive matrix
#

you guess?

fathom tendon
#

do you have the original problem

olive matrix
#

girlie you're the one who posted the question

bleak pier
fathom tendon
#

Peculiar

bleak pier
#

my answer by a formula will be e^2-e

worthy wing
#

Post the original question

bleak pier
#

someone asked me and i asked here

worthy wing
#

Your question doesn’t make sense

bleak pier
worthy wing
#

Lack of context and visibility

bleak pier
#

i am saying it is tending to infinity

humble torrent
#

ok so the sum $\sum_{k\geq1} \frac{2^{k-1}}{k!}$ ?

grand pondBOT
bleak pier
zealous schooner
bleak pier
humble torrent
#

Then a hint would be to maybe make use of the fact that $e^x=\sum_{k\geq 0}\frac{x^k}{k!}$

bleak pier
#

2^3-1

zealous schooner
grand pondBOT
bleak pier
humble torrent
zealous schooner
bleak pier
#

thanks

#

i want to know the process

humble torrent
bleak pier
#

i used a formula only

zealous schooner
#

The sum in question is $$\sum_{n=0}^{\infty}\left(\frac1{n!}\sum_{r=0}^{n-1}a^r\right)$$

grand pondBOT
#

kheerii

zealous schooner
#

To simplify the inside summation

#

And then use the identity $$e^x=\sum_{n=0}^\infty\frac{x^n}{n!}$$

grand pondBOT
#

kheerii

zealous schooner
#

Not quite, there’s one term missing

humble torrent
#

oh nvm theres a sum

#

in the numerator

#

God damn

zealous schooner
#

Indeed

midnight plankBOT
#

@bleak pier Has your question been resolved?

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midnight flame
midnight plankBOT
midnight flame
#

show that ^ for positive integers n

humble torrent
#

how much progress have you made?

midnight flame
#

um i tried it and then i think i did something completely wrong

#

so barelyt any

#

lemme give it one more go before asking sos

humble torrent
#

Yes

midnight flame
#

yeah so i'm unsure of what do to with RHS

humble torrent
#

my idea is to start with the LHS

#

by using the binominal theorem

midnight flame
#

i started with LHS and got 2 cos (cos + isin)

humble torrent
#

so set say x = cos(..) +isin(..)

midnight flame
#

binomials yeahh

humble torrent
#

then you want to expand (1+x)^n

#

you get $\sum_{k=0}^n \binom{n}{k}x^k$

grand pondBOT
humble torrent
#

see if this helps

#

maybe writing x as e^i(2pi/n) instead

midnight flame
#

nah haven't done euler's yet

#

meant to be solved w demoirves and binomials i reckon

humble torrent
#

ok so ignore eulers

#

if youre allowed demoivers it deosnt really matter

#

i used it as shorthand really here

#

but yeah try using that

#

then you "distribute" k into the arguments of cos and sin

#

if that even helps

midnight flame
#

ok thankss

#

.close

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#
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twilit field
#

$\frac{\left(1\cdot2\cdot3\dots2n\right)}{2^nn!}=\frac{2^n\left(1\cdot1\right)\left(2\cdot2\right)\left(3\cdot3\right)\dots}{2^{n\ }n!}$

twilit field
#

is this right?

grand pondBOT
#

ƒ(Why am. I here)=I don't Know

deep vine
#

no

hollow oyster
#

What are you trying to prove

twilit field
#

one minute

#

$\frac{(2n)!}{n! \cdot 2^n}$ is an integer

grand pondBOT
#

ƒ(Why am. I here)=I don't Know

twilit field
#

Induction would probably be enough here

#

now that I think of it

deep vine
#

just prove it directly

twilit field
#

👍 let me think of a way

burnt flame
#

think of double factorial*shifted double factorial

deep vine
#

(2n)!/(n! 2^n) = [ (n+1) (n+2) ... (2n) ] / 2^n
top has exactly n even numbers, so at least n factors of 2

twilit field
#

wait, doesn't the top have n/2 even numbers?

#

I reached until there too

#

but was stuck thereafter

deep vine
#

yeah mb

#

to be more specific

#

n/4 numbers divisible by 2 only, n/4 numbers divisible by 4

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that should do it

twilit field
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hmm

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yeah

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makes sense

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thanks

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.close

midnight plankBOT
#
Channel closed

Closed by @twilit field

Use .reopen if this was a mistake.

twilit field
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.reopen

midnight plankBOT
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burnt flame
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each additional term adds a factor of 2

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just as 2^n does

twilit field
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got it

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thank you

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.close

midnight plankBOT
#
Channel closed

Closed by @twilit field

Use .reopen if this was a mistake.

midnight plankBOT
#
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twilit field
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is this right?

midnight plankBOT
twilit field
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I can justify myself if need be

agile lichen
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yes but don't forget to prove the case n = 1

twilit field
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I'll justify myself, just a minute

subtle blaze
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$\frac{1}{1^2} + \frac{1}{2^2} \textcolor{red}{+} ... \textcolor{red}{+}\frac{1}{n^2}$

grand pondBOT
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Frosst

subtle blaze
twilit field
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oh

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sorry

subtle blaze
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it doesn't seem very obvious why it's true

agile lichen
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he's doing induction

subtle blaze
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i know he is

twilit field
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ok, not too sure of how I'd justify it

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it just feels right

dreamy lichen
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"We will prove it by induction on n:"
Base case:
blah blah blah blah. Base case works.

Inductive step:
Assume 1 / 1^2 + 1 / 2^2 + ... + 1 / n^2 <= 2 - 1/n. [Insert inductive step here]

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mention that you are using induction

twilit field
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ah, ok

dreamy lichen
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seperate base case and inductive step

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and thats pretty much all you will need to do

twilit field
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so what I'm doing is overkill

dreamy lichen
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no, it's unclear

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you are doing induction

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but in a very unclear way

twilit field
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got it

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thanks

agile lichen
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write up the whole proof and then post it here

twilit field
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👍

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done, let me TeX it

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just 2 minutes

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To prove : $\sum_{i=1}^{n} \frac{1}{i^2} \leq 2 - \frac{1}{n}$
\
base case:- $1 \leq 2-1$
\
inductive hypothesis :- $\sum_{i=1}^{n} \frac{1}{i^2} \leq 2 - \frac{1}{n}$
\
we now prove that the hypothesis holds true for the (i+1)th case
\
$\sum_{i=1}^{n} \frac{1}{i^2} + \frac{1}{(n+1)^2} \leq 2- \frac{1}{n+1}$

but $\sum_{i=1}^{n} \frac{1}{i^2} \leq 2 - \frac{1}{n}$
so saying
\
\
\
$2- \frac{1}{n}+ \frac{1}{(n+1} \leq 2 - \frac{1}{n+1}$ isn't wrong
\
\
solving this inequality we get this is true for all $n\in \mathbb{N}$
\
\
thus our hypothesis is true
\end{document}

grand pondBOT
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ƒ(Why am. I here)=I don't Know

twilit field
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I think I've made a errors when translating to TeX

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oops

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is this better

dreamy lichen
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so saying ___ isn't wrong

I'd definitely replace this with something like
"so it suffices to show that ___"

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Also, I feel like you are trying to avoid doing the proof that inequality, so instead you just said that it suffices to solve the inequality

twilit field
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I did the inequality in my notebook

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I got $n \neq n+1$

grand pondBOT
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ƒ(Why am. I here)=I don't Know

twilit field
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wait

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my bad

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$n \leq n+1$

dreamy lichen
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\leq?

grand pondBOT
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ƒ(Why am. I here)=I don't Know

twilit field
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which shows it's true for all n

dreamy lichen
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okay, but in a proof you'd usually write that in one line

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yk what i mean

twilit field
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yeah

dreamy lichen
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it's good to learn to "translate" between solving an inequation and doing that one-line proof

twilit field
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got it

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is the rest of the proof fine?