#help-23
1 messages ยท Page 41 of 1
the construction is using one of the quintessential properties of sups/infs over the reals
oh alr
can u elaborate pls
this is one of the key properties of sups over sets of real numbers
you use this exact property to construct the sequence you want
what sequence do i want tho
something that converges to sup A
the above property says there are elements in A which are arbitrarily close to sup A
yeah i get that
this is the same as saying a sequence converging to sup A exists
okay
thats all there is to it
layla has explained the core parts of the solution
but where's a or epsilon relevant in any of that
hmm okay ill try connecting it all now..
but how does $$
\text{sup}A - \frac{1}{n} \leq a_n \leq \text{ sup}A
$$ "formally" prove that $a_n$ converges to supA?
Levens
do an epsilon-N proof
with that? how
so then the question is
if you want an element of the sequence within say the 59714th decimal place of sup A
then you can go along 59714 elements of the sequence
and find the 59714th element
and by the way its been constructed
itll be within the 59714th decimal place of sup A
so basically to do an epsilon-N proof
wait why -k
?
usually you do if epsilon > 0 then there exists N such that ...
im just saying in the specific case epsilon is 0.0.....01 you can do something like this
and actually thats all you need
thanks.. for infA, can I choose $\epsilon = \frac{1}{n} +10$?
Levens
for inf you just switch all the inequalities around
oh
or if you want to think of it as decimal places
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thank u snow, helped a lot
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two families rent a helicopter. The cost for the first family of 5 was $520. The cost of the second family of 3 was $432.
Write an equation to represent the cost per person and the base cost.
@warm dune Has your question been resolved?
"Write an equation to represent the cost per person and the base cost."
This is what the directions say?
I think a system of equations makes more sense
@warm dune
yeah like f(x)=mx+b
Ah, okay. But first we need to find m and b
And we can do that by solving a system of equations
Do you know what I mean by a system of equations?
would i do 5x + b = 520?
oh ok
That's one equation, what's the other one?
3m + b = 432?
looks good
5m + (520-5m) = 520
Close, but you substituted one equation into itself
You want to substitute one into the other
Yep
2m + 432 = 520
2m = 88
m = 44
is that right?
5x44=220
520-300=220
so the equation is f(x) = 44x + 300?
yeah your right
thank you very much
Sure thing ๐
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hello?
@frail wraith Has your question been resolved?
Its correct.
Ima submit this and if Im wrong itll be on you
I really. need the points, no pressure
If you intend to submit that, you should define what F is
F is the function that represents the matrix.
Yep. Then thats fine
Thanks, if theres nothing more.
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@frail wraith Has your question been resolved?
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Can you do a quick check??
Yeah looks fine
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help
what problem
@bold void Has your question been resolved?
@bold void still stuck?
@bold void Has your question been resolved?
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Can someone explain all 3 of these questions? The process to get the roots of the function.
,rotate
Okay so
It all equal to 0
Then I put the biggest ^ number in brackets to be ^2?
Can you do that?
Solve for y
All of it equals 0
(Xยฒ)ยฒ-12xยฒ+32
Ye
X=0 x=0 x=4 x=8?
Hmm
How do i get the roots?
I'm confused ๐ can you help me factorise?
Setting x^2= y here, we get y^2-12y+32=0
Can you factorise that ?
For y?
So y is?
So x is?
So u square root them
Yessir
X is 4, can be negative 4
I mean 2 and negative 2
Sqrt 8 and negative 8
Can you do the next one?
Yeah
Great:)
Ty
It's just a code
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sasa..
Lol this channels gonna close soon
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...what even is going on here?
2x+y = 5
4x+z = ?
is this EXACTLY how the problem was given to you?
Substitution chaos
Yes
new variable just dropped
then there's so, so, so little information.
you literally cannot say anything meaningful.
What?
if all you know about the variables x, y and z is that 2x+y = 5, then you cannot say anything about the value of 4x+z
should have gotten your hands on that to begin with tbh
im guessing you mistook some number with ? or something
Gosh dangit i forgot a y
๐๐๐๐๐๐๐๐๐๐๐
Let me try again
...
well i mean like. yeah.
but also taking this sort of thing directly from Google is sus
never thought google maths practice was a thing
fun fact, you could've just simply multiplied the whole original equation by 2 and that would've saved you a lot more time
Which one? Theres two of them?

2x + y = 5
multiply by 2
4x + 2y = 10
you are done
Ann thats a two btw
Not a z
Lol
i gathered as much
Anyways with 2 instead, would it still be solvable?
Anyways
This one is interesting
And yk i have an evil devious idea
yes, and lex even already told you how.
this is really bad on many levels
Why?
the first of which is that you actually went out of your way to write $2 \divisionsymbol 3$ instead of $\frac{2}{3}$ (and likewise in two other places)
and the second line is simply nonsensical
Ann
granted, the problem itself was also written poorly; it should have been $$\frac{2}{3} x - \frac{3}{2}x = \frac{1}{12}$$
Ann
How do you know its (2/3)x and not 2/3x?
you mean how i knew it wasn't $\frac{2}{3x}$?
Ann
i had to make an assumption. you were studying linear equations so i assumed the equation you posted here was also meant to be linear.
Yeah that
Isnt 2/3x linear too?
I mean no exponents= automatically linear
Hmmmm may i ask why?
allowing x into the denominator puts you in the wider class of rational equations, which can get considerably more complicated in their behavior than linear ones
i mean for example $\frac{1}{x} + x = 5$
Ann
this is not linear anymore
Ohh thats scary
May i ask about the 2nd line?
you can try to ask me but i will be unable to answer except to repeat that it is nonsensical
no,
there is no such thing as "transposing"
you did some symbol manipulation that ultimately does not mean anything
like
ok no nevermind
there is nothing more i can say here.
I guess i just butchered math so badly lol
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here is my process for finding the derivative of 1 over x
apparently the end result should be negative 1 over x squared instead of positive 1 over x squared
why is this?
surely it should be -x^2 instead of -{1 over x squared} though right?
ah
wait
yeah i think i get why now
The minus out front and the minus in the power aren't interacting in any way
Perhaps you thougut they were cancelling somehow
nah i was just being silly and not knowing where to move the negative sign but its the same if its on the bottom or on the outside
alright thanks you two ๐
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is inf^0 = 1?
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can i add and remove pi from something like cos(3pi-x) like if its in a form of +2kpi sorry if my vocabulary is bad i study maths in french
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@reef galleon Has your question been resolved?
<@&286206848099549185>
@reef galleon Has your question been resolved?
you can add and subtract 2pi because cosine is periodic
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sin^2 A + cos^2 A = 1
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<@&286206848099549185>
nvm
Nice
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I am completely lost on this question right now, Three vectors, x,y, and z satisfy x+y+z=0 . Calculate the value of x. y+y.z+x.z if lxl =2 , lyl=3 and lzl=4
I've tried applying properties and manipulating the formula, but nothing works and i don't know what to do
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honestly i just need help working through it bc i have no idea how to solve it
then i have 1 more like this
Find such numbers when you multiply them it becomes 60 and when you add them it becomes 23
Yes
Here's a video on how to factor when a isn't equal to 1
https://www.youtube.com/watch?v=f0GqRY1ORbQ&ab_channel=BrianMcLogan
๐Learn how to factor quadratics when the coefficient of the term with a squared variable is not 1. To factor an algebraic expression means to break it up into expressions that can be multiplied together to get the original expression.
To factor a quadratic trinomial where the coefficient of the term with a squared variable is not 1, we find tw...
ok thank you
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A 43-foot rope is tied to the top of a 31-foot vertical cliff face. When pulled so that it just reaches the ground, what is the angle of elevation to the top of the cliff?
This is how I interpreted the angle
U can construct a right triangle with this (triangles are every which is awesome, I know). What does it look like? The height of the cliff and the rope belong to which of the legs and hypotenuse?
Oh yeah
Now apply trig stuff. Soh Cah toa stuff
I did Tan -1 (43/31) but it got marked as wrong.
So I'm trying Cos-1(31/43) But I don't know if I'm interpreting the triangle in the problem properly.
the triangle is correct, you choice of trig function is not
So Sin-1 (31/43)?
One is the o and the other is the h in soh Cah toa. Yes u r correct
yes
Thats weird, we didn't use Sin at all for any of these problems.
Sin = opposite side / hypotenuse
Ye I know.
use whatever the problem calls for
46.13
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A 125-foot tower is located on the side of a mountain. In addition, a 170-foot-long guy wire is attached to the top of the tower which is anchored at a point 55 feet downhill from the base of the tower.
What is n โ A
Apply law of cosines
Do you know law of cosines?
Partially
Maybe this will help
https://www.youtube.com/watch?v=3wGQMyaWoLA&ab_channel=ProfessorDaveExplains
The law of sines helped us solve for all the sides and angles in SAA, ASA, and SSA triangles, but now we have to learn the law of cosines if we want to solve for SAS and SSS triangles. This one is kind of like the pythagorean theorem, just for oblique triangles instead of right triangles, so it has trig functions in it. Let's check it out!
Watc...
This one right?
Being the law of cosines
Would be writing
170^2 = 55^2 + 125^2 - 2(55)(125) cos C
?
Not quite
Wrong placing?
Notice how the angle you are finding is equal to the opposite side. Like in this diagram, side c is across angle c
So here, is 170 across from angle a?
No โฆ itโs 125
Yes
Technically the angle you are looking for is angle A, so across from angle A is side A
I just noticed that your problem labeled that angle as A
And not C
You would just need to swap some numbers around
Yeah
Thank you!!! A LOT
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Uhhh where would I start with this problem, the limit goes to 0, and Iโm pretty sure I canโt do the integral test, all Iโve learned is p-series, integral test, comparison test, divergence theorem, and smth else, I havent learned the ratio test tho, any help?
@lean otter Has your question been resolved?
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@lean otter Has your question been resolved?
u can use comparison test
O okay Iโll try Ty
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hello can someone help me with this?
@lament flare Has your question been resolved?
whatd you try?
The letter c
@lament flare Has your question been resolved?
yes
@lament flare Has your question been resolved?
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this is my global minimum when it asks for the coordinates for the x value would i write it between my domains like for exmaple (x,-20) where x is 20<x<40
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@flint python Has your question been resolved?
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Can anyone have alternative solution for this question. Q) If x=a(cost+tsint) and y=a(sintโtcost), find dx^ 2/ d ^2y โ
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if a man has to work 5 days in 31 days, without working 2 consecutive days, how many arrangements are possible for his days of work?
if they have to work for 5 days, how many days must they not be working?
then can you simplify the problem in terms of arranging working and non-working days?
it can
starter: how many ways can you fit the first working day into the block of non-working days?
it works but you need to try it the right way, it's a trick
wdym
there are 5 working days
you get that the man has 26 non-working days
how many ways can you insert 1 working day into the 26 non-working days
then, while satisfying the condition that the man is not working 2 consecutive days, a second working day
26 for the first one, then 24 for the second one...
no
but i dont see the point
this is very difficult
the point is that this generalizes for any number of working days :p
instead of computing directly, try to fit working days between non-working days
why? why are we fitting working days into non working days? to avoid having 2 consecutive days of work? if yes how are we achieving that
no, they are flexible like that
you imagine slots in between
and unimagine them back after choosing
I dont get what you mean
like if you have two non working days and you add a working day
.X.X.
the dots are where you can fit it
WXX, XWX, XXW
and this solves the problem in one step
if you;re slotting many working days at once, they will automatically be non consecutive
it's a trick, iv just hoped you'd invent it
I still have a hard time understanding this
here the X represents non working days?
yeah
when you add a second day, the dots will literally stay like that
so it's a one step choice
but you removed the dots
i dont get why the dots are necessary
we're just starting with 27X and then seeing how many ways we can fit a W between 2 X right?
this is correct?
Ah i see the point of the points
you would think it's literally stars and bars but I don't see a way to make it like that
what made you think of this emthod
i want to understand how we went on to think about this
someone probably spoiled it to me i don't remember
so there is nothing that can help me in future problems, some pattern to spot
i was helpless in front of this one
so how does it work?
I can see like, we're distributing 26 in 4 stretches
and then also into 5, twice
and then also into 6
no it's 5 bars, give me a second, my brain is dying .-.
it's 5 bars yeah
ok it's 5 bars and 22 stars, that took too long to wrap my head around
you have 5 bars and 26 stars
but 4 of the bars can't directly follow each other, so you sub these 4 bars by a shift of 1
and you get 5 bars and 22 stars
26โ4 yeah
is this something i should know :/
great, a wiki article
it's a neat trick that will save you a lot of time if you get it into memory
when i was doing math comp and undergrad combinatorics we had a saying "if it looks too hard transform it into stars and bars"
looool
yeah that's lame i know, i don't know youtube links
lool no i wasnt sarcastic i spend most of my time on wiki
but wait didnt we actually transform this problem to make it easier
like it isnt easy, but we seem to have added a spot to make it easier
no?
our month became 32 days
of which we need 31
I just cant see how this overwhelming problem is suddenly a simple one
and instead of picking the days of work and non work... we instead fixed the days of non work and simply bothered with our days of work right?
the not-so-rigorous answer is that you transform the problem in a way that you won't be counting in a way where previously arrangements drastically changes count formula
if you try to brute force by saying "okay day 1 can be a working day, then 3,5,7,9, then if you fix 1,3,5,7 you have 23 more choices" you get massive nested sums
wdym previously arrangements drastically cahnges count formula
That 1,3,5,7 and 1,3,5,8 are drastically different?
if you try to count by "if work day 1 is here how many ways can i place work day 2, then how many ways can i place workday 3"
you can immediately see that the number of ways to place say, workday 3 depends on placements of workday 2 and workday 1 in a not very nice way (and gets much worse for days 4/5)
so you want to either 1. make the dependency less nasty or 2. hopefully skip the dependency thing altogether
another way to see it for dazzle:
a sequence of working and non working days is a sequence of w and x, where w are working and x non working
you have 5 w and 26 x
but two w can't be next to each other
so we make a little transformation: we couple 4 of the x with 4 of the w, to form couples Y = (x, w), and now Y can follow w and Y can follow each other
now we have 27 symbols: 1 w, 4 Y, 22 x where w and Y are of the same object "working day to place in the sequence"
yes
the method frog introduced, where you insert work days into the 26 non-work days, makes the dependency less nasty (you just get 27 * 26 * 25 * 24 * 23, and you can determine how many ways to insert more)
stars and bars skips that dependency altogether, by observing the structure:
there must be at least 4 non-work days between the first work day and the last work day, since no work days can be consecutive
therefore when we say 5 bars and 22 stars, it's equivalent to treating there being only 22 non-working days, and removing the constraint that workdays have to be non-consecutive, because we implicitly add 4 more non-work days, one after each of the first 4 work days
yeah thats a cool way to put it
yeah i mean just how did the method frog used removed dependency what did he notice
The structure is a very cool thing ill look for in future problems
just incredible
what frog's method does is it ignores the order of the work days
because when you try to brute force, you label work days "first work day", "second work day (after the first)", etc
by inserting workdays between non-workdays you're saying one of the workdays can be here, .. etc
also that method to count under the restriction of no two identical items together is one of the earlier tricks you'll learn in combinatorics probably in the first lecture or two
worth remembering because it's easy to apply
Okay
how did you get so good at combinatorics
you seem to have understood it to its core
did math comps back in middle/highschool and read a decent amount of brilliant/art of problem solving
Talk math and math contests like MATHCOUNTS and AMC with outstanding students from around the world. Join our active message boards now.
are they free?
artofprobemsolving is
aops is, brilliant no
depends tbh, aops has problems + people who gives solution
like you did
some people are clearer than others
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statistics question: Can you have an estimate with no bias and high variance?
doesnt "no bias" imply ur estimate is the same as the population parameter?
so for example if i want to estimate the mean of something using a sample data what does it mean for my sample mean to have bias zero?
no bias means the expected value of your estimate is the real expected value for your parameter
and in that case is it possible to have no bias and also high variance?
yes, the impossible task is to have both no bias and low variance
why would it have high variance? high variance means if i get another sample data i would get a different result right?
and if i keep repeating the results will differ largly
high variance means the probability of getting data far from the expected value is high
like a flat distribution instead of a condensed one
but doesn't no bias mean that what im estimating will be close to what is real?
nvm is ee
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How to prove AD bisect angle BAC?
AD is diameter so ABD and ADC are right triangles and share two sides since BD = DC
angles in semi circle?
I don't understand what you're trying to say
I'm talking about similar triangles
ABD and ADC are congruent
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how would i go about multiplying a rotation martix for the Xs and Ys here? i havent learned aboht matrices yet but im trying to understand what im supposed to do when given such a thing
just out of pure curiousity
so im supoosed to multiply the x with every single one of the matrix elements in Rx?
is it supposed to be a matrix with xyz coords and then i would need to multiply it like 2 matrices would be multiplied
$\begin{bmatrix} 1 & 0 & 0\ 0&\cos{\theta}&\sin{\theta}\end{bmatrix}$
Right
ah so its literally just 2 matrices being multiplied
that was the only thing i didnt undestand, whether it was supposed to be just x multiplied, but that would be weird and not propper, so i was confused as to what exactly to multiply
thx
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hey guys
I was tasked with solving the diff equation, f''+f=(cosx)^2
I managed to arrive at the solution f=1/2-1/6(cos(2x))
however, im not sure if those are indeed all the solution, not am I sure how to compute it to account for all possible solutions
can anyone calrify this for me ?
Go to #โhow-to-get-help, this is someone elseโs channel
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notice that it's always negative
i meant the root
it wont touch at y = 0
thats for sure
u want to solve
yep
?
Modus
or what
yes
then think about it - can it achieve positive value? (I mean function)
the x can be positive
well, other question, can sqrt( something ) be negative?
i solved it on a calculator, just wanted to know how the process is with such functions
no
if i remeber
yes
let's consider it's even zero, then if we subtract 4 we'll definitely get negative value
what about complex values
wait, you're asking about complex? ;d
just generally asking
substract 4 from what
isn't it add ?
from 0 or negative value
in other words you have:
$$\sqrt{4-x^2}=-4$$
Modus
you've said square root is always NON-NEGATIVE
on LHS u have negative 4
so is there any x such that equation is true?
this equation is false
ok with functions that dont touch at y = 0
how do we do it ?
i imagine this parabole touches -4
or something
y = - value
@glass carbon do I do trial and error to see which x value the function stops ?
.
If the function has no zeroes
Then it has no zeroes
Full stop
But it never touches y = 0
ok so I have to trial and error with the y
i know
maybe formulate your question properly
Are you thinking about y-intercept or what
domain maybe??
the points -4, -2 ; -4, 2
ok domain
thats the one that gives these points
it works
thanks
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Can someone help me with this? My professor wants it done with Plurality with Elimination AND Pairwise Comparison .
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I donโt really understand what the function g does
By g(f) is it referring to a general function f in B_n?
But then what is f(1)โฆ representing?
f is in Bn yes
It's based on the idea that an n-periodic function is uniquely defined by its first n values
So this associates the function to these values
And proves it's a bijection
Could you explain this a bit more, Iโm not following when you say by itโs first n values
Say you want to find f(k).
You can just use euclidian division to write k = qn + a
Then because f is n-periodic, f(k) = f(a), with a in [1, n[.
Therefore f is entirely defined by its first n values. The rest is found by repeating it
I see, thanks. Essentially this g function is just mapping f to its n tuple so f(1)โฆf(n) before it repeats
What is f and f over score though?
I also donโt really get how it is injective
Since take for example a function f with f(1) to f(5) as (2,4,6,2,8,)it has period 5 but, f(1)=f(4)
You don't want f to be injective. You want g to be injective
They took f, f_ as the two functions for injectivity. I agree it's a very weird notation
I see on the very last line shouldnโt it also be f over score(p)
But I donโt really see how I would efficiently come up with this in an exam haha, are there any tips to do so?
Yes. That proves f = f_, and thus that g is injective
Honestly this is pretty straightforward I'd say
How do you describe periodic functions ? First by their period. That gives you the union of the Bn
Then you know that it is sufficient to prove each Bn is countable (or uncountable since it's an open question)
Then a simple understanding of how periodic functions behave lets you find this function very quickly and guess that it's a bijection. Then checking it isn't difficult either
So if you understand the material, I wouldn't say there's a single hard step for coming up with this proof. I came up with the same thing before looking at the solution and took me a few seconds. Which is not meant to be boastful, but to argue that it's quite a straightforward proof for a countability problem
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<@&286206848099549185>
@fiery escarp Has your question been resolved?
What's the first part of the question that you find confusing?
I computed the wronskain determinant and got -3
So Iโm not sure how to get the dimensions
As far as I can tell, the Wronskian determinant (that I had to look up, because I hadn't seen it before) isn't necessary to solve this problem.
I was thinking the dimensions of the subset h is 3
I do get that dim(H) is 3.
And for part b I said it is a basis for p2
And for part c I got some fractions then o I donโt know what to do
What is dim(P_2)?
The question doesnโt say the dimensions of p2
Hm. I guess I can explain it a different way too. You said that the three polynomials are a basis for P_2, what does that mean?
(I'm looking for a particular idea, or an equation, that follows from the definition of what a basis is.)
What is the span of a basis?
Why did you say for part b that it is a basis for P_2?
(As in, what led you to decide that.)
Hereโs my working out
Wait so it doesnโt spam p2
So itโs not a basis for p2
FYI it's -4, not 4, for the matrix you wrote in the middle there.
What's the standard basis for P_2?
In this case, it does span P_2.
The standard basis for P_2 is {x^2, x, 1}. It shouldn't be too hard to convince yourself that span({x^2, x, 1}) will give you every polynomial of degree at most 2. Therefore dim(P_2) = 3.
How I solved the problem: Since dim(H) = dim(P_2) = 3 and H is a subspace of P_2, this means that H = P_2. This makes answering b and c easy.
@fiery escarp Has your question been resolved?
Does this help with solving c
I... can't deduce what you're doing for most that image.
Oh
The only thing I could recognize was the matrix made of the three polynomial's coefficients (with 4 instead of -4).
I'm confused how to answer c
If you have that H = P_2, and the three polynomials are a basis of P_2, what is a basis of H?
If you have that H = span({three linearly independent polynomials}), what is a basis of H?
Hint that gives away what I tried to get you to remember/say: ||A basis of V is a set of linearly independent vectors (in this case polynomials) that span V.||
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so for (a), I got 1 as my gcd using the euclidean algorithm. How do I apply that to question (b)? Do I just solve for s or t?
mhm
so 1 = 75 - 37 x 2?
The following error occured while calculating:
Error: Undefined symbol x2
Result:
1
seems legit
but how do I know what the values of s or t are based on this?
for the negative? yes
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What would 5x+6=56 be?
um
first u want only x on 1 side
im guessing u r solvin forx
so we can start with -6 on both sides
what do u get after that?
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They are all discrete I think @plain gate
yeah seems right
how ?
Well it's not how much gas it is, it's the number of litres
Which can be any positive real number, not just integers
same with the age and weight, they are continuous
Idk the task seems extremely ambiguous.
I disagree
I think you could go real brainiac and count the number of molecules of gasoline and say it's discrete
But that seems counter to the spirit of the question
And the age and weight are unquestionably continuous
Looks good
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So i did (a) and (b), got 1 as the gcd for (a), and s = -2 and t = 1 for (b)
for question (c), I did 1 == 75(1) + 37(-2) (mod 75) and since 75 mod 75 is 0 I'm left with
1 == 5(-2) (mod 75)
oh wait
is 10 supposed to be the answer
where did the 5 comes from?
that's a good question why did I write 5
uh
let me go fix it
ok so
what i have now is
1 == 37(-2) (mod 75)
-74?
nope
what's the definition of an inverse (mod n)?
or really any multiplicative inverse
37(x) == 1 (mod 75) right
yeah
well 37 and 75 could be any number
so what's the inverse of 37 (mod 75)
yes
one question
or 73 if you want to make it positive
why is -74 mod 75 1?
another way to think about it is a and b are equivalent (mod n) if a = b + kn
yep
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@toxic stratus hi
for the characteristic equation is it just lambdaA - I
how do i find lamda
characteristic equation is det(A-Lambda*I) = 0
hi
o
nihao
where do i spawn in lambda tho
or is it an arbitrary value
oh wait
it is
arbitrary
Two snows?
yes
e-vals
egyenvalues right?
ye
nice
lemme do it rq
wait
its det(A - lambdaI) = 0
can it be
det(lambdaI - A) = 0
doesnt really matter since its just a possible sign change
o
if you do lambda first the coefficient in front of the largest power of lambda will always be 1
how do i type lambda in latex