#help-28

1 messages · Page 292 of 1

white karma
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nice

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so what does the theorem tell us?

slate glacier
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oh wait, I corelated it with the power, whoops

white karma
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np

slate glacier
white karma
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Right, so here a=7 and n=11 as you mentioned. So first we should compute φ(11)

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and what's the value of that?

slate glacier
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okay, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, so 10

white karma
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Right, in general φ(p)=p-1 for any prime number p since all positive integers <p are relatively prime with p

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Ok so now we know that 7^10\equiv 1 mod 11

slate glacier
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yes

white karma
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But we want to know what 7^102 is mod 11

white karma
slate glacier
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can we raise both sides to the power 10 (if that works here)?

white karma
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Nice, and this works

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you can see that it works like this: 7^100=(7^10)(7^10)...(7^10)\equiv 1x1...x1\equiv 1 mod 11

slate glacier
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yes I see that

white karma
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nice

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ok so using this, what is 7^102 mod 11?

slate glacier
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might seem insane, but multiply 49 on both sides?

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and it's 44 + 5, so is it 5?

white karma
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Nice it's 5

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So 7^102=7^100x7^2\equiv 1x49\equiv 5 mod 11

slate glacier
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oh wow, this is good

white karma
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Yeah this simplifies things nicely

slate glacier
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Okay wait, how about I do 77^77 mod(8) step by step, and you correct me if I mess up

white karma
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Sure, that's the spirit. After all if you don't try to do it yourself you won't gain much

slate glacier
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okay, so we have 77^77 mod(8)

white karma
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mod 8

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just so that you don't start calculating and realize this later

slate glacier
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and we have euler's formula, a^(relative primes of n) equi 1 mod(n)

white karma
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Nice but remember the condition

slate glacier
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so here, a is 77, and n is 8

white karma
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There should be something satisfied between a and n right?

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Otherwise you can't use euler's theorem

slate glacier
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oh yeah, was it that n can't be a divisor of a?

white karma
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No

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that a and n are relatively prime

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ie gcd(a,n)=1

slate glacier
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1

white karma
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Nice

slate glacier
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okay okay, so a = 76 and n = 8 wouldn't have worked?

white karma
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Yeah Euler's theorem probably wouldn't have worked here

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but you could've done something else

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Note that 76=19x4

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and 4²=16\equiv 0 mod 8

slate glacier
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ooooooo, yes I see what you mean

white karma
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so 76^k\equiv 0 mod 8 for any k>=2

slate glacier
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oh wait yeah

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okay so coming back to the steps

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So a = 77, n = 8

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function(8) = 4

white karma
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correct

slate glacier
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so 77^4 equi 1 mod(8)

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both sides to 19th power

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77^76 equi 1 mod(8)

white karma
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Nice

slate glacier
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77^77 equi 77 mod(8)

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77 = 72 + 5

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Holy shit

white karma
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Equivalently, 77=80-3

slate glacier
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ahhhh yes yes

white karma
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But yeah did you see how easy the question becomes using this theorem

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Hahaha

slate glacier
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sure man, damn

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Can I ping you if I have some stuff related to this later, as I have to go somewhere

white karma
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Sure, ping me whenever you want and I reply whenever I can

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I probably won't reply in the next few hours since it's 2:40 am here so I will go sleep after a few mins

slate glacier
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Alright man, thanks for your help🙏

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@turbid badge thanks to you too man

white karma
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np, I hope this was helpful and not much boring.

slate glacier
white karma
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Btw are you doing this for a course in uni like discrete math?

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Or are you just self studying

slate glacier
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I've got an exam on 23rd

white karma
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Ohhh I see

slate glacier
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We'll talk later, can I add you?

white karma
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Yeah sure

slate glacier
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Alright, see ya man, and thanks again🫡

white karma
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Cya, gn

slate glacier
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.solved

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twin wolf
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what is your doubt

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whats your doubt

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<@&268886789983436800>

spiral vigil
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.close

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scarlet dome
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scarlet dome
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2c

silk bridge
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what have you tried ?

scarlet dome
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Man

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I tried everything

silk bridge
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you calculated net Interest for 1st year

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do for 2nd

scarlet dome
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Yes I did

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I am confused

silk bridge
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silk bridge
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what did you do ?

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what is the formula for compound Interest

scarlet dome
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Which one to use as principal

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The net int of 1yr - tax + principal of 1st year or

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CI of 1st year + principal of 1st year

scarlet dome
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Alright

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Thankd

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S

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.close

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silk bridge
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i have doubt mostly net

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what is the answer

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thick hedge
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thick hedge
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$ \frac{1}{\sqrt{2π}} \int_{0}^{\infty} \frac{x^r}{x} e^{ - \frac{\ln^2(x)}{2}}dx$
\Let $\ln(x)=t$
\$ \frac{1}{\sqrt{2π}} \int_{-\infty}^{\infty} e^{tr} e^{ - \frac{t^2}{2}}dt$

glossy valveBOT
thick hedge
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unless I'm missing something , there's no neat way to integrate this?

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Would finding the MGF be a better idea here

thick hedge
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.close

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spice pilot
#

Is the following claim true? If $B(x,r)\subset B(y,s)$ and $r<s$, then $\overline{B(x,r)}\subset B(y,s)$. Here's my attempt: Let $z \in \overline{B(x, r)}$. Then, $\forall \epsilon > 0, \exists p \in B (x,r)$ such that $d(p, z) < \epsilon$. Pick e.g. $\epsilon=(s-r)/2$. Hence, $$d(y,z) \leq d ( p, z ) + d ( y, p ) < s + \epsilon.$$ How do we proceed?

glossy valveBOT
split hatch
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The claim is false

spice pilot
split hatch
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B(1/2, 1/2) = (0, 1) is a subset of B(1, 1) = (0, 2) but the closed ball \bar B(1/2, 1/2) = [0,1]

spice pilot
split hatch
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not really

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replace B(1, 1) with B(3/4, 3/4) in the example above

spice pilot
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vernal rose
#

I have a line that goes from (1, x1, 0) to (0, x2, 1). Looking from (0,0,0), How can I solve, which point in the line I have "steepest" angle to?

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Please show the original problem, exactly as it was stated to you, with the entire original context. A picture or screenshot is best. If the original problem is not in English, then post it anyway! The additional context might still be helpful. Do your best to provide a translation.

turbid badge
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hmm i guess it'd be whichever point is farther to the origin

silk bridge
turbid badge
vernal rose
turbid badge
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i forgot the meaning of steep 😭

turbid badge
vernal rose
vernal rose
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I am trying to find an point with angle closest to 90 degree from origo.

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(For extra context: I am trying to find a single point to rotate towards instead of rotating towards x and then z... and I have hard time finding the single point)

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y is the height axis in this picture just for clarification

silk bridge
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then find perpendicular to the lie from orgin

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it will be steepest

vernal rose
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Huh ... was it that simple : DDD

silk bridge
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if you want steepest

vernal rose
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Oh I think it is

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Yes thank you 🫶

silk bridge
vernal rose
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vernal rose
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.reopen

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vernal rose
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I have doubts

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For line that goes through (0,0,1) and (1,1,0) the steepest point is (1,1,0), which is not perpendicular to the line from origin

full forumBOT
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@vernal rose Has your question been resolved?

silk bridge
vernal rose
silk bridge
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ok

vernal rose
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Highest this value in the line (y / sqrt((x-z)^2 + (z-x)^2) 😄

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My Bachelor's degree was over third just math courses and I can barely remember anything

silk bridge
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i'm not university

vernal rose
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You are not an university

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student

silk bridge
silk bridge
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y here is 1

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but idk how to find vecotr equation of a line

vernal rose
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In the picture y is 1. In reality y can be anything.

silk bridge
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yh but we need to find for given

silk bridge
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by finding maximum

vernal rose
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Yeah I just forgot how to find maximum value. But I think that it is easily googleable... so I try that

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Hmm it's very doable. I can simplify that even further. Then maybe do derivative on it or something similar.

silk bridge
vernal rose
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x range is from 0 to 1. and then the line has points (0,x1, 1) and (1, x2, 0). x1 and x2 are some constants.
Then I can try to generate function that tries to find the maxima of (x * x1 + (1 -x) * x2)/sqrt(x^2 + (1-x)^2), if given the x1 and x2 to it.

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I have to go now, so I let the bot close this

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Good chance to solve this with this amount of knowledge thumb_rat

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.close (Huh it might be that the x = (x1 / x1+x2) have to double check when I get back)

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digital void
#

,tex
Hello. I am trying to prove the following statement. Is the proof correct?
$\forall \ \text{sequences} \ x_n , \ \lim_{n\to\infty} x_n = \infty : \ \lim_{n\to\infty} f(x_n) = l \implies \ \lim_{x\to\infty} f(x) = l $

\vspace{0.23cm}
Proof: We assume that the negation holds. So:
$$ \left(x_n \to \infty , f(x_n) \to l \right) \land \left( \lim_{x\to\infty} f(x) \neq l\right) $$

\vspace{0.25cm}
Meaning: $$ \forall M_1 >0 , \exists n_0 \in\mathbb{N} : n\geq n_0 \implies x_n >M \ (1) $$
$$\forall \epsilon >0 , \exists n_0 : n\geq n_0 \implies \ x_n >M \implies \lvert f(x_n) -l\rvert <\epsilon \ (2) $$
$$\exists \epsilon ' >0 , \ \forall M>0 : \exists x: x>M \ \text{and} \ \lvert f(x)-l\rvert >\epsilon \ (3) $$
So, take $\epsilon = \epsilon ' $ on (2) and choose a sequence $x_n >M , n\geq n_0 $. We then have that (2) and (3) hold simultaneously (for $x=x_n $ in (3)). Perchance.

glossy valveBOT
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fijokazż

digital void
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i forgot the epsilon' at the end of (3) btw

rapid rain
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"You can't just say Perchance"

thorny isle
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You can't choose your sequence. You know that it exists but nothing more. So you use said sequence from the begining. You have the right idea. There exists $\epsilon >0$ such that for all $m \in \mathbb{N}$, there exists $n\ge m$ such that $|f(x_n) - l | > \epsilon$. Now, take $M>0$, there exists $N \in \mathbb{N}$ such that $\forall n \ge N, x_n \ge M$. You will be able to conclude by taking $M \in \mathbb{N}$ and applying the first statement.

glossy valveBOT
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bourbactive

digital void
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up to | f(x_n)-l|>ε?

rapid rain
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Statement (1) I presume

digital void
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oh ok

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im very confused ngl

rapid rain
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I'll add that the proof as you wrote it is incomplete. You do need to explicitly create a sequence $(x_n)$ such that $\lim_{n\to\infty}x_n = \infty$ (by create I mean show how you pick each term) and $\lim_{n\to \infty}f(x_n) \neq l$. So yes the idea is to apply statement (3) to increasingly bigger values of $M$.

glossy valveBOT
#

Rafilouyear2026

digital void
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can we not create our x_n via induction?

rapid rain
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you could, but why do induction when you don't need it particularly

digital void
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im not sure i know any other way

rapid rain
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so apply it to M = n for all n

digital void
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okay i think i see it

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x1>1 , x2>2 ...

rapid rain
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$$\forall n\in \bN : \exists x_n: x_n>n : \text{and} , \lvert f(x_n)-l\rvert >\epsilon'$$

glossy valveBOT
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Rafilouyear2026

digital void
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why do we need it to hold for all n naturals?

rapid rain
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you pick x_2 with property (3) applied to M = 2...

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etc

digital void
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oh okay thats easier to see

rapid rain
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so you pick x_n with property (3) applied to M = n, for any n

digital void
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okayokay

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how do we connect it to (2)?

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like, make them be true simultaneously

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we take ε= ε' and M=n for all n

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so we dont need the n_0 i assume

rapid rain
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I noticed that statement 2 doesn't make sense

digital void
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oh lol

rapid rain
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where does the M come from

digital void
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(2) is true for any positive real, so we take that number to equal M

rapid rain
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The definition of $\lim_{n\to\infty} f(x_n) = l$ is the definition of sequence converging to some finite value, with $u_n = f(x_n)$

glossy valveBOT
#

Rafilouyear2026

digital void
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uhm

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okay lemme write it

rapid rain
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$$ \forall \epsilon>0 , \exists n_0 \in\mathbb{N} : n\geq n_0 \implies |f(x_n)-l|<\epsilon$$

glossy valveBOT
#

Rafilouyear2026

digital void
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yes but we also have that x_n >M in the middle

rapid rain
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??? what is M

digital void
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okay lets not call it M

rapid rain
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we don't need what the limit of x_n is to talk about the limit of f(x_n) btw

digital void
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$$ \forall \epsilon>0 , \exists n_0 \in\mathbb{N} : \forall N>0 \ n\geq n_0 \implies x_n >P \implies |f(x_n)-l|<\epsilon$$

glossy valveBOT
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fijokazż

rapid rain
#

it makes less and less sense 😭

digital void
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huhhh

rapid rain
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why do you need to involve this "x_n > P" when it isn't needed

digital void
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but for f(x) diverging we say x>P => d(f(x),l)< ε

digital void
rapid rain
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diverging?

digital void
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sorry

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converging

rapid rain
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here f(x_n) is a sequence

digital void
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wha

rapid rain
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sure, it's formed using the function f, but it doesn't matter

digital void
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f(x_n) is a function that takes the terms of a sequence as inputs

rapid rain
digital void
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fr?

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oh

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but we dont have (f(x_n))_n

rapid rain
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?

digital void
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u wrote it like that just now

rapid rain
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what do you think $\lim_{n\to\infty} f(x_n) = l$ means

glossy valveBOT
#

Rafilouyear2026

rapid rain
#

what is "f(x_n)" here

digital void
rapid rain
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and not a sequence

digital void
rapid rain
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I can't do much if you can't see that "$\lim_{n\to\infty} f(x_n)$" is the limit of the sequence $u_n = f(x_n)$, as $n$ goes to $\infty$

glossy valveBOT
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Rafilouyear2026

digital void
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i can see your point

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but i always thought that was a function

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im kinda flabberghasted

rapid rain
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f is a function, but $\lim_{n\to\infty} f(x_n)$ isn't the limit of the function as the input goes to $\infty$

glossy valveBOT
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Rafilouyear2026

digital void
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yeah that makes sense tbh

rapid rain
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For example

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take $f(x) = |x|$

glossy valveBOT
#

Rafilouyear2026

rapid rain
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and $x_n = (-1)^n$

glossy valveBOT
#

Rafilouyear2026

rapid rain
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what is $\lim_{n\to \infty}f(x_n)$

glossy valveBOT
#

Rafilouyear2026

digital void
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1 prolly

rapid rain
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well not "probably", exactly

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each term f(x_n) = 1

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so it's a constant sequence, so it converges to that value

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did we require at any point that x_n converges to some limit

digital void
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we didnt

rapid rain
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and if so, did we use the limit of $f(x)$ as $x$ converges to that point

glossy valveBOT
#

Rafilouyear2026

digital void
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x_n didnt converge anywhere so thatd be pointless

rapid rain
# digital void we didnt

ok, so the definition of $\lim_{n\to\infty} f(x_n)$ shouldn't require any knowledge about the limit of $x_n$ and/or how $f$ behaves generally around the limit of $x_n$

glossy valveBOT
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Rafilouyear2026

digital void
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yeah that makes sense

rapid rain
glossy valveBOT
#

Rafilouyear2026

digital void
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okeoke

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so i gotta retry the entire exercise

rapid rain
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now, of course, if you also know that $x_n$ also goes to $\infty$. Then for any $M$ and $\epsilon$, you can find a rank $n_0$ such that you have both $x_n > M$ and $|f(x_n)-l| < \epsilon$

glossy valveBOT
#

Rafilouyear2026

rapid rain
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but the definition of "f(x_n) goes to l" doesn't depend on that fact

digital void
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hmm okay

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that doesnt hurt the proof then cuz we dont need one to imply the other

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ill try to do it and ill come back

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thank you for all of that i had no idea i was doing it wrong

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.close

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twin wolf
#

hello where does the part underlined in red come from

rapid rain
#

so, you're looking for $F(u,v) = A\begin{pmatrix}u\v\end{pmatrix} + B$

glossy valveBOT
#

Rafilouyear2026

rapid rain
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where A is a 2x2 matrix

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and B is 2 x 1, representing F(0,0)

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Say I want the bottom-left edge of R to be mapped to the leftmost edge of D

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then F(0,0) = (1,1)

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that solves for B

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then, pick enough points to completely fix F

twin wolf
rapid rain
#

turns out you need two more points

rapid rain
#

so suppose $G(u,v) = F(u,v) - B$ is linear for some $B$

glossy valveBOT
#

Rafilouyear2026

rapid rain
#

Then, since we're in finite dimension, $G(u,v) = A\begin{pmatrix}u\v\end{pmatrix}$ for some matrix $A$

glossy valveBOT
#

Rafilouyear2026

twin wolf
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so are we writing the parallelogram D as some linear combination of vectors?

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and B is some corner we pick

rapid rain
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D is the image of R through an affine transformation

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so each vector/point in the square R is mapped to some vector/point in D

twin wolf
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so can we construct the parallelogram using F(u.v) = <x,y> = <x0,y0> + u <a0,a1> + v <b0,b1>

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and i can pick x0 and y0 as a corner on the parallelogram

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say 1,1

rapid rain
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yep

twin wolf
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and u and v vectors are direction vectors im assuming

rapid rain
#

uhhhhh

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they're not vectors

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they're the coordinates of (u,v)

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your original point

twin wolf
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oh

rapid rain
#

but if you make "u" bigger, meaning moving horizontally in the positive direction,

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that corresponds to moving in direction (a0,a1) by that much on the parallelogram

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and changing v, meaning moving vertically when you're on the square

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corresponds to moving in direction (b0,b1) on the parallelogram

twin wolf
#

so i have the vertices
(1,1)
(2,0)
(3,2)
(2,3)

i can pick a0,a1 to be say from (2,0) to (1,1) and subtract these to get <-1,1>
and then b0,b1 to be from (2,3) to (1,1), subtracting we get <1,2>

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these are the edges of the parallelogram?

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so F(u,v) = (1,1) + u(1,-1) + v(1,2) maybe

rapid rain
twin wolf
#

ohh yeah

rapid rain
#

and you get <1,-1>

rapid rain
twin wolf
rapid rain
#

exactly the F they got

twin wolf
#

ohhh okayyy thank you 🙂

rapid rain
#

Notice we could have picked a different edge to map (0,0) to, and we could have decided to switch (a0,a1) and (b0,b1) together

#

that would result in different transformations that still map R to D

#

but it shouldn't really impact your computation of the integral in the end

twin wolf
#

ohh so like we could have picked (2,0) to be x0,y0

rapid rain
#

you'll make a minus appear in the determinant, because you inverted the orientation of the space

twin wolf
#

if F doesnt preserve the mapping orientation

rapid rain
#

uh

#

yeah the formula should have a |det| right

twin wolf
#

yea like |JF(u,v)|

rapid rain
#

uh

#

no |JF(u,v)| means det(JF(u,v))

twin wolf
#

i have written in my notes | JF(u,v) | = | (x_u)(y_v) - (x_v)(y_u) |

#

| (x_u)(y_v) - (x_v)(y_u) | and the stuff inside this is the stuff you get from calculating the determinant

#

ad - bc stuff

rapid rain
#

very weird notation, I wouldn't recomment

#

either write |det(JF(u,v))| or smthg like ||JF(u,v)||, idk

#

no not that either, will be mixed up with the norm

twin wolf
#

this is what it says

rapid rain
#

my body is reacting viscerally to that, but ok

twin wolf
#

lmao is this not normal notation i js started this topic so idk

rapid rain
#

The determinant of a matrix A (no absolute values) can also be written as |A| in linear alg

twin wolf
#

ya

rapid rain
#

so

#

|A| = ad - bc in a lot of courses

rapid rain
twin wolf
#

then how would you indicate absolute value if u cannot use | | signs

rapid rain
#

write det(...) for the determinant

#

|det(...)| becomes clear then

#

If your book tells you to write the absolute value of the determinant of the jacobian as |JF(u,v)| then I won't put it on you for following the book's notation

#

but I will definitely put it on the book

twin wolf
#

it says detDF bla bla

rapid rain
#

huh

#

the "jacobian" for them is the determinant of the differential????

twin wolf
#

uh idk catgiggle

#

is the jacobian the derivative matrix or

rapid rain
#

well the jacobian usually refers to the jacobian matrix

#

and the jacobian determinant is not referred to as the jacobian

#

Also the notation 💀

#

$JF(u,v)$ or $J_F(u,v)$ also usually refers to the jacobian matrix

glossy valveBOT
#

Rafilouyear2026

twin wolf
#

oh idk then Lol

rapid rain
#

It's not your fault

#

your book just decided to go wild on notations and go against all notation conventions generally accepted

twin wolf
#

lmao

rapid rain
#

so on your own exam, you'll be fine using the book's wild notation system

#

but when talking with other people, expect the same concerns to be risen

twin wolf
#

i have another book that shows this stuff

#

hold on

rapid rain
#

ok so $\frac{\partial(x,y)}{\partial(u,v)}$ is also the jacobian determinant and they also call it "Jacobian"

glossy valveBOT
#

Rafilouyear2026

rapid rain
#

Am I the crazy one?

#

I'll ask helpers

twin wolf
rapid rain
twin wolf
#

i see

#

maybe for undergrad level they just interchange

rapid rain
#

But the notation JF(...) for the determinant is still wild to me

twin wolf
#

hello higher!

rapid rain
#

interchanging notations between a matrix and its determinant is the scariest thing ever

twin wolf
#

anyways thanks for the help!

#

.solved

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upbeat venture
#

how would i type this into a ti 30 calculator

upbeat venture
#

i keep getting the wrong exponent

#

its supposed to be like 2.60x10^-9

#

but i keep getting like 2.60x10^-49

frank gale
#

did you type 10^17

upbeat venture
#

no i typed 10^-17

frank gale
#

hmmcatthink

lime trellis
#

,w calculate ((6.626*10^(-34))(3(10^8)))/(7.65(10^-17))

glossy valveBOT
lime trellis
#

just spam brackets

torn jolt
#

Hi

upbeat venture
#

OH

#

WAIT ILL TRY THAT

#

HIIII

torn jolt
upbeat venture
#

.close

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muted token
#

Where am I going wrong here?

full forumBOT
robust slate
muted token
#

I should be getting $-\frac{x}{12y^{14}}$

glossy valveBOT
#

Vortac

grizzled basin
#

$-\frac{3}{36}=-\frac{1}{12}$

glossy valveBOT
#

JamR_71111

grizzled basin
#

just needed to reduce the fraction a bit, everything else is good

muted token
glossy valveBOT
#

Vortac
Compile Error! Click the errors reaction for more information.
(You may edit your message to recompile.)

robust slate
#

I'm ngl I thought that was an 8 cause it was so zoomed out lol

muted token
#

I need to remember to always reduce

grizzled basin
#

yeah, easy to forget sometimes

muted token
#

I was thinking somehow 36 needed to be in numerator and 3 in divisor

#

thanks everyone!

grizzled basin
#

Np 😁

muted token
#

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timid dune
#

Can i get some help on this?

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timid dune
stiff pier
timid dune
#

this is my attempt to draw this

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@timid dune Has your question been resolved?

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opal wolf
full forumBOT
opal wolf
#

In the proof , why do we require |xn|< 1+ |x|

#

Can't we just write , M to be Supremum of {|x1|,....,|xn| }

#

Since if, M is the Supremum of absolute value

#

All |xn| ≤ M

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@opal wolf Has your question been resolved?

split hatch
split hatch
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random sky
#

If every closed and bounded subset of a metric space $M$ is compact, does it follow that $M$ is complete?

glossy valveBOT
#

Kasli, the Scourge

random sky
#

Allow me to write up my solution to this problem. I want somebody to review it to make sure there are not flaws in my reasoning.

#

This may take a few minutes.

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@random sky Has your question been resolved?

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#

Kasli, the Scourge
Compile Error! Click the errors reaction for more information.
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#

Kasli, the Scourge

random sky
#

That took a lot longer than I anticipated.

#

Perhaps now is also a good time to mention that the definition of compactness I am using is that of sequential compactness.

atomic valve
#

i think it's good modulo typos

random sky
#

I apologize for any typos in the solution.

#

My TeX is not very good yet.

atomic valve
#

your TeX is good

foggy vapor
#

But anyways its fine

random sky
#

Guh. Always the little mistakes.

#

Anyhow, thank you Lex and _gmz for reviewing this.

#

I'll close the channel now, if there are no other concerns.

#

.close

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foggy vapor
atomic valve
#

well they didn't but it's true

foggy vapor
foggy vapor
random sky
#

Thank you.

atomic valve
#

i suppose if you were to spell out why cl(B_r(x)) ⊆ B_(r+1)(x) it would be the same amount of work to show cl(S) ⊆ B_(r+1)(x) already

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shut copper
#

Find the number of integer solutions, where each 0 <= x_i <= 10

x_1 + x_2 + x_3 +x_4 = 21

shut copper
#

i know i'll have to use inclusion-exclusion

#

but im having trouble framing it

split hatch
#

Let A_k, 1≤k≤4, be the number of solutions where x_k > 11, ...

shut copper
#

so we have to find S - (A_1 and A_2 and A_3 and A_4)

split hatch
#

not and, ∪

#

but yes

shut copper
#

so the final expression would be

#

|S| - (|A1| + |A2| + |A3| + |A4|) + ( |A1 U A2| + ...) - (|A1 U A2 U A3| + ... ) + |A1 U A2 U A3 U A4|?

split hatch
#

The unions should be changed to intersections

#

Here's one thing to note

#

Is it possible that two variables exceed 11 at the same time?

shut copper
#

no

#

so we only consider the first part

#

thank you

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charred carbon
#

There are 14 identical metal cubes in terms of shape and size. Out of these, 7 cubes are made of aluminum and the other 7 are made of steel. It's known that all the aluminum cubes weigh the same, all the steel cubes weigh the same, and that aluminum cubes are lighter than the steel ones. You have a balance scale. The least number of weighings needed to guarantee to correctly identify which cubes are aluminum and which are steel is

charred carbon
#

My mind is kinda fried rn, I can't think of anything

grave elm
#

There is surely a lower bound of 8 (here is a hint on how to obtain it: consider the n.o. possible distributions of the aluminium cubes and the number of possible "states" you could obtain by using the scales n times)

charred carbon
silk bridge
#

two cubes at a time on balance scale

charred carbon
#

Don't have to be one each side

silk bridge
#

ok'

grave elm
#

if we do 1-1 comparisons, either:
one is heavier one is lighter -> we are done
they are the same weight -> we put them aside

We can do this 7 times, at worst we will have 6 pairs put aside. Then we can form pairs of those pairs in 3 comparisons and at most 2 pairs of pairs will be equal. Last comparison solves that.

So that's an upper bound of 11.

charred carbon
#

I get the upper bound

charred carbon
grave elm
#

it actually has a wikipedia

charred carbon
#

The distributions thing is messing up my mind

charred carbon
#

to me

grave elm
#

it seems like a pretty difficult problem

charred carbon
#

breadpensive It's supposed to be a uni entrance exam problem, shouldn't be that hard

charred carbon
grave elm
grave elm
#

do they perhaps have a file with intended solution somewhere

charred carbon
#

But computer is not

charred carbon
#

Hmm, I have a different problem to ask

#

Each cell of a 100×100 square grid is colored with one of 20 colors. A cell is considered 'lonely' if its color is different from the color of every other cell in the same row and column. The maximum number of 'lonely' cells that can exist on the grid is

charred carbon
#

pandathink as my friend suggested, we color 2 diagonal lines first , it's possible to make all of the them "lonely" so the lower bound is 200 for now

#

Now to color each column or row is tricky, I don't have a good strategy for that

#

-# if these kind of questions keep appearing on the test I might be cooked 🥀

grave elm
#

,calc 100C19

glossy valveBOT
#

The following error occured while calculating:
Error: Undefined symbol C19

brazen lark
#

wrong number sorry

grave elm
#

We pick 1 color which will just color whatever remains, so we wont expect this color to ever be lonely. We are left with 19 colors to use on lonely cells, my idea is that we can color 19 cells in each row s.t. they dont repeat in columns

#

that'd give 19*100 lonely cells

#

the question is, whether thats possible

charred carbon
#

Yes

grave elm
#

put 1,2,3,4,5,6,...,19 in the first row

#

then shift by one in the row under it

#

shift by one again, ...

brazen lark
#

ohh

charred carbon
#

Yes I just thought of tha

brazen lark
#

thats a famous way to color

charred carbon
#

It seems possible

brazen lark
#

i think so

grave elm
#

the question is whether its optimal

charred carbon
#

pandathink hm

grave elm
#

it feels like it both should and shouldnt be

charred carbon
#

try add one more color to that, and see if we can get one more "lonely" cell

charred carbon
grave elm
#

the issue is that if we try to do the same thing with all 20 colors in a row, suddenly we have no way to color the rest of the row

#

without eliminating one color by using it

#

putting us back to 19

#

the usage of rows in this strategy is already perfect, if anything could be optimized, its the usage of columns

#

actually, per each row there can be at most 19 lonely cells

#

which proves optimality (100 rows * 19 cells per row)

#

so 1900 is indeed optimal

brazen lark
#

i think this is kind of sudoku

charred carbon
#

How come I didn't think of that

grave elm
#

im asking the same question, + "earlier"

#

its very simple, but there are just so many possible ways to approach the problem, which look very promising that the trivial one isnt so easy to find

charred carbon
#

19*100

#

I instantly thought this's wrong

grave elm
#

what uni is this for btw? Something with math? Is it like a very prestigious uni or sth? Those question seem quite non-standard for an entrance exam

charred carbon
#

Ha Noi University Of Science and Technology

brazen lark
#

WOWZ

charred carbon
#

I meant it's the top 1 engineering uni

grave elm
#

Oh, interesting

charred carbon
#

In my country

brazen lark
#

that kinda question is high school entrance test here

grave elm
charred carbon
#

Idk, I still have another question

#

One more to go

brazen lark
#

letsgo

grave elm
charred carbon
#

There are 100 passengers waiting in line to board a plane with a maximum capacity of 100 people. The seats on the plane are all numbered from 1 to 100. Alice, the first passenger in line, forgot her seat number, so when she boards, she randomly chooses a seat. Each passenger boarding after Alice will sit in their assigned seat if it is available, or choose a random seat if their assigned seat has already been taken. What is the probability that the 100th passenger will sit in their assigned seat (seat 100)?

charred carbon
#

My answer is 1/2 pandathink

#

Just want a clarify

brazen lark
#

wait im doing it

grave elm
#

so we just need to find the probability that someone sits back to alice's seat

grave elm
#

i cant even find a way to phrase it correctly

charred carbon
brazen lark
#

yeah

#

i get 1/2 too

grave elm
#

If someone went and picked alice's original seat to sit to, then they'd close the cycle and everyone else would just get to seat themselves normally

#

i might be wrong tho

silk bridge
brazen lark
#

it depends wether the 99th person chooses the right seat or the wrong seat

brazen lark
silk bridge
brazen lark
#

wait hmmmm

silk bridge
#

Wait north/south

brazen lark
#

nein

grave elm
#

oh i kinda get the 1/2

charred carbon
#

I don't know how to phrase my argument correctly 🥀

silk bridge
grave elm
#

say alice's og seat was 1 and the 100th guy's seat is 2. Then at the 100th guy's turn, exactly one of 1 and 2 are free. But they are both "not-my-seat" to all the passengers 2-99. So by symmetry, both have the same probability of being taken

brazen lark
grave elm
#

and that probability must therefore be 50%

brazen lark
#

oh yeah makes sense

grave elm
#

its basically because from the perspective of passengers, alice's and last guy's seat are indistinguishable

charred carbon
#

But my answer is correct so

#

yay

brazen lark
#

yay

grave elm
# charred carbon By symmetry is not so satisfying to me tbh

It's just that whether or not the 100th guy gets to sit at his seat is fully determined by whether some previous passenger takes alice's seat or his seat. And from perspective of that some previous passenger, those seats are exactly the same. So he just chooses randomly between them

silk bridge
charred carbon
silk bridge
silk bridge
brazen lark
#

?

silk bridge
grave elm
silk bridge
brazen lark
charred carbon
#

So he is the last person to choose a wrong seat

grave elm
#

but what if 3rd guy chooses 100th guys seat?

charred carbon
#

I meant

#

still in between them everyone would be in their seat

grave elm
#

oh, right

#

yeah, that works

charred carbon
#

yay

grave elm
#

so the only unsolved question is the metal cubes one, which I have no idea how to do

brazen lark
#

i know how to slove if theres one but 7 makes me dizzy

charred carbon
#

okay doin hw here is kinda fun, 1 more igcatgiggle

#

Consider four real numbers (not necessarily distinct), each with an absolute value not exceeding 1/2, and the sum of any three of the four numbers is an integer. What possible values can the sum of the four numbers take among the following options?

#

-4/3

#

2/3

silk bridge
charred carbon
#

1/3

#

0

#

Multiple correct answers question

grave elm
#

0 is obvious

charred carbon
#

-4/3 and 0 are trivial

#

What about 2/3 and 1/3

grave elm
#

suppose exactly 3 numbers are same, then
x + x + x and x + x + a are both integers, meaning that a and x differ by an integer, requiring x = 1/2 and a = -1/2 or vice versa

suppose exactly 2 and 2 numbers are same, then
x + x + y, y + y + x are both integers, meaning that x - y is also an integer, again requiring them to be 1/2 and -1/2

suppose exactly 2 numbers are same, then
x + x + y, x + x + z, x + y + z must both be integers, requiring y - z to be an integer, so y = +-1/2, z = +-1/2 and hence x = 0 by the last one

#

none of these can produce 2/3 or 1/3

#

so they'd have to be all distinct

brazen lark
#

wowx

charred carbon
grave elm
#

x + y + z, x + y + w is an integer would require z - w to be an integer

grave elm
#

i.e. z and w must again be +- 1/2

#

simiarly x and y

#

so u cant get 2/3 or 1/3

charred carbon
grave elm
#

oh im being stupid here

#

either z and w are +-1/2 or z = w

#

oh, which means they're the same, which I already dealt with

brazen lark
#

yeah thats

grave elm
#

if it's 0, they are the same (so not distinct)

#

if it's 1, by the absolute value condition it must be 1/2 - (-1/2)

#

if its -1, it must be -1/2 - (1/2)

charred carbon
#

pandathink oh interesting

brazen lark
#

NICE

grave elm
#

Surprisingly, the only non-trivial cases which can produce something other than combinations of halves are the trivial ones - where all the numbers are the same

charred carbon
#

Wao that's impressive

brazen lark
grave elm
#

i was just expecting the distinct case to be difficult

#

so i did the casework first, but it probably wasnt needed

charred carbon
#

catthink you shouldn't

brazen lark
charred carbon
#

We have 40 questions in the span of 60 min

brazen lark
#

wow

silk bridge
silk bridge
#

1.5 min/

brazen lark
#

we have 7questions per 100min

grave elm
#

whats the avg score?

#

i can imagine spending 5 days on the og question and still not getting it

brazen lark
#

lol

charred carbon
#

Average is 50-60/100

#

like 75% taker

grave elm
#

and are most of the questions easier than the ones u sent?

silk bridge
brazen lark
#

lol he is

charred carbon
#

others are mid

#

Not so easy tho

brazen lark
charred carbon
#

Probably take sometime

#

Like 3D geo

charred carbon
silk bridge
#

avg asian shit 😢

charred carbon
#

And the precision

brazen lark
#

you know my questions

silk bridge
charred carbon
brazen lark
#

yeah ill send you the pdf

silk bridge
#

what is your exam called again @charred carbon

charred carbon
silk bridge
#

imma check some past year papers if available

charred carbon
#

I don't think there's any in english

silk bridge
#

btw are all q solved

#

for this channel

charred carbon
#

This's a not so creative question from my test

#

Consider three spheres (S1), (S2), (S3) with radii R_1 = 11, R_2 = 13, R_3 = 19, each externally tangent to the others. A plane (P) intersects the spheres in three circles with centers A, B, C, respectively, and the centers of the three spheres all lie on the same side of (P). It is also known that the three circles of intersection have the same radius.

Given that AB² = 560, the value of AC² is .....................

brazen lark
#

hmm thats interesting

charred carbon
#

Given a regular quadrilateral pyramid S.ABCD with a base edge of 24 and a height of 20. Let G1, G2, G3, and G4 be the centroids of triangles SAB, SBC, SCD, and SDA, respectively, and O be the center of the square ABCD. The volume of the pyramid O.G1G2G3G4 is ....................

#

Another one

brazen lark
#

oh thats science

charred carbon
#

That's interesting

brazen lark
#

this is math

charred carbon
#

Oh that's not many problems I see

brazen lark
#

but it needs a lot of creativity and fast thinking

charred carbon
#

Interesting

#

Mine is overwhelming

#

That kinda sucks

brazen lark
#

ill be back in 15 minutes

silk bridge
silk bridge
#

so pi^2 = Ri^2 - r^2

charred carbon
#

you can try that out, imma get some sleep

#

It's bad that on average I sleep 5 hours/day or even less

#

Gotta fix it

#

.close

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#
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silk bridge
#

i suck at 3d

#

but distance b/w center of S1 and S2 = R1 + R2
now we relate that to AB

#

idk how

charred carbon
silk bridge
#

AB is r^2

charred carbon
#

3D geo here is tedious

#

.reopen

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charred carbon
#

Here letmme show you something

silk bridge
#

🫂

silk bridge
#

you would get r

charred carbon
brazen lark
#

what do i need to get

silk bridge
brazen lark
#

i love geometry

charred carbon
#

Just an example kek

silk bridge
#

oh

charred carbon
silk bridge
#

😢

silk bridge
charred carbon
#

Oh that's not a hard problem catgiggle , the hard one has even more lines than that

#

My nightmare back when I prepared for comp

silk bridge
#

true

#

scary

charred carbon
#

I don't understand why people love geo

charred carbon
#

.close

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#
Channel closed

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brazen lark
silk bridge
charred carbon
#

yeah

silk bridge
#

i dont remember

charred carbon
#

I'm fionna

silk bridge
#

who fionna

charred carbon
#

recal recall

#

@tall onyx

#

I sent that pic

silk bridge
#

ohk

charred carbon
#

Sometime

#

When people say

silk bridge
#

right

charred carbon
#

I love geo

#

I throw them that pic

brazen lark
#

its beautiful tho

#

lol

charred carbon
brazen lark
#

ouch

silk bridge
#

do you have multiple personalities or something (light joke)

brazen lark
#

lol

charred carbon
#

Someone told me to be a girl on that account

brazen lark
#

what

silk bridge
charred carbon
#

Not lance lmaooo

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opal wolf
#

If (xn) and (yn) be sequences of real numbers that converge to x and y respectively.

Show that the sequence (cX) converge to cx , for some c ∈R

opal wolf
#

My idea:

#

This is just rough idea

errant tiger
errant tiger
# opal wolf

ur method is correct but u need to do it more neatly ig

#

first u need to construct a delta and then find the epsilon

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@opal wolf Has your question been resolved?

opal wolf
#

I have to make it formal

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broken rover
#

Let n be a natural number such that n+1 is divisible by 24, and let d be any positive divisor of n. Prove that d^2 - 1 is divisible by 24 and that the sum of all positive divisors of n is also divisible by 24.

broken rover
#

i call k=n+1/d and replace n with kd but im js stuck there, i think mod can be used in some ways but idk how :<

grave elm
#

n+1 is divisible by 24 means n = 23 = -1 (mod 24)

#

to prove that d^2 - 1 is divisible by 24, we need to prove d^2 = 1 (mod 24)

#

for that, I'd consider mod 3 and mod 8 separately

fallen scroll
#

agh got sniped KEK

charred carbon
#

-# sugoi Celine is backkk

broken rover
broken rover
grave elm
torpid vault
grave elm
#

and hint #2: Q1 can be useful

broken rover
grave elm
#

for example
24
1, 24/1
2, 24/2
3, 24/3
4, 24/4

#

this is the only "symmetry" of divisors, so that's the motivation for trying it

broken rover
broken rover
#

.close

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twin parrot
#

An old lady went to the chemist shop to buy medicines. She fell short of the total amount required but the kind shopkeeper sold the medicines at a loss of 12% so that she could buy them. If the cost price was ₹2200, find the selling price.

onyx glen
#

are you sure you didn't miss anything

#

there's not enough info right now

twin parrot
#

That's the thing

#

That's why I can't solve it

onyx glen
#

take a pic of the original question

#

just to be sure it isnt your fault

twin parrot
#

I don't have a phone rn

onyx glen
#

so you swear your life on the fact that it says "she fell short" and not "she fell Rs. 999 short"...

twin parrot
#

Yeah

#

Ok leave it I guess the book had a mistake

onyx glen
#

yeah, i was going to say you should skip.

twin parrot
#

thx for the help btw

#

bye

slate violet
onyx glen
onyx glen
silk bridge
#

he has left...

onyx glen
#

he's not left the server as far as i can see

silk bridge
#

i mean offline

onyx glen
#

blergh.

#

well, let his channel time out then

slate violet
#

.close

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#
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slate violet
onyx glen
#

@twin parrot

#

nope he's still in the server

#

just choosing to ignore me

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twin parrot
onyx glen
#

thank you for editing it out as i asked.

twin parrot
#

I really left the chat

#

I guess it was a glitch that made it like this

onyx glen
#

anyway ok

#

if you have nothing else to ask then you can .close the channel

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void jay
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void jay
#

is my approach solid or would you solve it in a faster way

#

also, i was hesitant regarding the movement of decimals to eventually get to 1+4 = 5 in the exponent

silk bridge
spiral vigil
void jay
#

ok thanks

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opal wolf
#

I proved in a different manner . Since we had already proved the theorem : if xn≥yn , then limit(xn)≥ limit (yn)

My proof :-

From hypothesis, we have
xn≤yn for all n ∈N
Theorem 3.2.5 implies ,lim(xn)≤lim(yn) Also
yn≤zn for all n ∈N
Theorem 3.2.5 implies ,lim(yn)≤lim(zn).

Since lim(xn)=lim(zn), we have lim(xn)= lim(zn)≤lim (yn)

Combining the inequalities,
lim(zn)≤lim (yn) and lim(yn)≤lim (zn)
We have lim(yn)= lim(zn).

So that lim(xn)=lim(yn)=lim(zn).
QED

fast peak
#

your proof assumes that lim y_n exists

#

that is a strong assumption

#

one strength of the squeeze theorem is that you do not need that extra assumption

#

the theorem itself shows you that lim y_n exists

opal wolf
#

Doesn't it help

fast peak
#

but lim y_n doesnt have to exist in general

#

-1 <= (-1)^n <=1 but lim (-1)^n doesnt exist

#

you have to prove that such a situation doesnt happen here

opal wolf
fast peak
#

you are writing lim(yn)

#

how do you know that you can write that

#

how do you know that it is a number

opal wolf
#

Okay so first we have to prove that yn is convergent

#

I missed that part

opal wolf
fast peak
#

yes you have to prove that it is convergent

#

and then your proof works

opal wolf
#

Ok thank u

opal wolf
fast peak
#

they show -eps < y_n-w < eps

#

which essentially directly shows that y_n converges to w

#

which is immediately what you want

opal wolf
fast peak
#

well its not like you have to follow it but essentially all proofs you see have been optimized over decades/centuries and you can rarely improve them

opal wolf
#

I was just trying to find an alternate proof

#

Seems like , it diverts to books prove ,...only

#

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sturdy valve
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sturdy valve
#

not able to start

fleet radish
#

Bayes‘ theorem?

silk bridge
#

classic bayes

hidden vault
#

yup bayes

sturdy valve
#

cn u do without that

hidden vault
hidden vault
sturdy valve
#

huhh alr fine then

#

thx

#

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sand badge
#

Pls explain 2nd degree 😭

hidden vault
#

u mean ax^2+bx+c ?

sand badge
#

Nah

#

Tf is that

hidden vault
#

what is 2nd degree then

#

also pls open a channel first in help for a question

silk bridge
#

!help

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To ask for mathematics help on this server, please open your own help channel or help thread. See #❓how-to-get-help for instructions.

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