#help-28
1 messages · Page 292 of 1
oh wait, I corelated it with the power, whoops
np
that for a^(number of relative primes of n), the remainder of it when divided by n is 1?
Right, so here a=7 and n=11 as you mentioned. So first we should compute φ(11)
and what's the value of that?
okay, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, so 10
Right, in general φ(p)=p-1 for any prime number p since all positive integers <p are relatively prime with p
Ok so now we know that 7^10\equiv 1 mod 11
yes
But we want to know what 7^102 is mod 11
We will use this fact
can we raise both sides to the power 10 (if that works here)?
Nice, and this works
you can see that it works like this: 7^100=(7^10)(7^10)...(7^10)\equiv 1x1...x1\equiv 1 mod 11
yes I see that
oh wow, this is good
Yeah this simplifies things nicely
Okay wait, how about I do 77^77 mod(8) step by step, and you correct me if I mess up
Sure, that's the spirit. After all if you don't try to do it yourself you won't gain much
okay, so we have 77^77 mod(8)
and we have euler's formula, a^(relative primes of n) equi 1 mod(n)
Nice but remember the condition
so here, a is 77, and n is 8
There should be something satisfied between a and n right?
Otherwise you can't use euler's theorem
oh yeah, was it that n can't be a divisor of a?
1
Nice
okay okay, so a = 76 and n = 8 wouldn't have worked?
Yeah Euler's theorem probably wouldn't have worked here
but you could've done something else
Note that 76=19x4
and 4²=16\equiv 0 mod 8
ooooooo, yes I see what you mean
so 76^k\equiv 0 mod 8 for any k>=2
oh wait yeah
okay so coming back to the steps
So a = 77, n = 8
function(8) = 4
correct
Nice
Equivalently, 77=80-3
ahhhh yes yes
sure man, damn
Can I ping you if I have some stuff related to this later, as I have to go somewhere
Sure, ping me whenever you want and I reply whenever I can
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
np, I hope this was helpful and not much boring.
It was great man, thank you
Btw are you doing this for a course in uni like discrete math?
Or are you just self studying
I've got an exam on 23rd
Ohhh I see
We'll talk later, can I add you?
Yeah sure
Alright, see ya man, and thanks again🫡
Cya, gn
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2c
what have you tried ?
!show
Show your work, and if possible, explain where you are stuck.
Which one to use as principal
The net int of 1yr - tax + principal of 1st year or
CI of 1st year + principal of 1st year
net
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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$
wai
unless I'm missing something , there's no neat way to integrate this?
Would finding the MGF be a better idea here
I mean isn't this the nth MGF of the standard normal dist
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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?
psie
The claim is false
Ok. That's good to hear.
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]
If we demand that y in B(x,r), the claim becomes true, right? 🙂
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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?
can you rephrase
!xy or
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.
hmm i guess it'd be whichever point is farther to the origin
or is perpendicular to the origin?
that would be 90° which is the most, but ig we'll know when the ambiguity is clear
If I have line that goes through (1, 1, 0) and (0, 1, 1) then the point for steepest angle is in the middle of those points
wdym by steep closer to 90?
i forgot the meaning of steep 😭
probably what it should mean
oh
Yes!
I am trying to find an point with angle closest to 90 degree from origo.
(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)
y is the height axis in this picture just for clarification
Huh ... was it that simple : DDD
if you want steepest

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✅ Original question: #help-28 message
I have doubts
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
@vernal rose Has your question been resolved?
which plane do you want to maximize it with
The "ground" plane. I call it xz (and height is y).
ok
Highest this value in the line (y / sqrt((x-z)^2 + (z-x)^2) 😄
My Bachelor's degree was over third just math courses and I can barely remember anything
i'm not university
yh
but you have to maximize this angle
y here is 1
but idk how to find vecotr equation of a line
In the picture y is 1. In reality y can be anything.
yh but we need to find for given
if you find vecotr equation of the line then you just have to maximize this
by finding maximum
Yeah I just forgot how to find maximum value. But I think that it is easily googleable... so I try that
Hmm it's very doable. I can simplify that even further. Then maybe do derivative on it or something similar.
oh, yh hm.. you find critical points and check for maxima
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.
I have to go now, so I let the bot close this
Good chance to solve this with this amount of knowledge 
.close (Huh it might be that the x = (x1 / x1+x2) have to double check when I get back)
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,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.
fijokazż
i forgot the epsilon' at the end of (3) btw
"You can't just say Perchance"
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.
bourbactive
what exactly is the first statement here?
up to | f(x_n)-l|>ε?
Statement (1) I presume
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 $M$.
Rafilouyear2026
can we not create our x_n via induction?
you could, but why do induction when you don't need it particularly
im not sure i know any other way
by property (3), you can fix your epsilon'
so apply it to M = n for all n
$$\forall n\in \bN : \exists x_n: x_n>n : \text{and} , \lvert f(x_n)-l\rvert >\epsilon'$$
Rafilouyear2026
why do we need it to hold for all n naturals?
? you pick x_1 with property (3) applied to M = 1
you pick x_2 with property (3) applied to M = 2...
etc
oh okay thats easier to see
so you pick x_n with property (3) applied to M = n, for any n
okayokay
how do we connect it to (2)?
like, make them be true simultaneously
we take ε= ε' and M=n for all n
so we dont need the n_0 i assume
I noticed that statement 2 doesn't make sense
oh lol
where does the M come from
(2) is true for any positive real, so we take that number to equal M
The definition of $\lim_{n\to\infty} f(x_n) = l$ is the definition of sequence converging to some finite value, with $u_n = f(x_n)$
Rafilouyear2026
$$ \forall \epsilon>0 , \exists n_0 \in\mathbb{N} : n\geq n_0 \implies |f(x_n)-l|<\epsilon$$
Rafilouyear2026
yes but we also have that x_n >M in the middle
??? what is M
okay lets not call it M
we don't need what the limit of x_n is to talk about the limit of f(x_n) btw
$$ \forall \epsilon>0 , \exists n_0 \in\mathbb{N} : \forall N>0 \ n\geq n_0 \implies x_n >P \implies |f(x_n)-l|<\epsilon$$
fijokazż
it makes less and less sense 😭
huhhh
why do you need to involve this "x_n > P" when it isn't needed
but for f(x) diverging we say x>P => d(f(x),l)< ε
i think it is implied
diverging?
but that's for a function converging
here f(x_n) is a sequence
wha
sure, it's formed using the function f, but it doesn't matter
f(x_n) is a function that takes the terms of a sequence as inputs
(f(x_n))_n is the sequence of outputs of x_n by the function f
?
u wrote it like that just now
what do you think $\lim_{n\to\infty} f(x_n) = l$ means
Rafilouyear2026
what is "f(x_n)" here
for all epsilon >0 and for all P>0 : x_n >P => d(f(x_n),l)< epsilon
So you think lim(f(x_n)) = l interprets "f(x_n)" as a function
and not a sequence
im not sure what wed call it anymore
yeah
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 $\infty$
Rafilouyear2026
i can see your point
but i always thought that was a function
im kinda flabberghasted
f is a function, but $\lim_{n\to\infty} f(x_n)$ isn't the limit of the function as the input goes to $\infty$
Rafilouyear2026
yeah that makes sense tbh
Rafilouyear2026
and $x_n = (-1)^n$
Rafilouyear2026
what is $\lim_{n\to \infty}f(x_n)$
Rafilouyear2026
1 prolly
well not "probably", exactly
each term f(x_n) = 1
so it's a constant sequence, so it converges to that value
did we require at any point that x_n converges to some limit
we didnt
and if so, did we use the limit of $f(x)$ as $x$ converges to that point
Rafilouyear2026
x_n didnt converge anywhere so thatd be pointless
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 $x_n$
Rafilouyear2026
yeah that makes sense
So we just define $\lim_{n\to \infty}f(x_n) = l$ as we would do with any sequence $(u_n)$, applying it to $u_n = f(x_n)$
Rafilouyear2026
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 $|f(x_n)-l| < \epsilon$
Rafilouyear2026
but the definition of "f(x_n) goes to l" doesn't depend on that fact
hmm okay
that doesnt hurt the proof then cuz we dont need one to imply the other
ill try to do it and ill come back
thank you for all of that i had no idea i was doing it wrong
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hello where does the part underlined in red come from
You want to find an affine transformation to describe how R is mapped to D
so, you're looking for $F(u,v) = A\begin{pmatrix}u\v\end{pmatrix} + B$
Rafilouyear2026
where A is a 2x2 matrix
and B is 2 x 1, representing F(0,0)
Say I want the bottom-left edge of R to be mapped to the leftmost edge of D
then F(0,0) = (1,1)
that solves for B
then, pick enough points to completely fix F
hi rafilou!! im a little confused where this comes from
turns out you need two more points
An affine transformation is a linear transformation up to some additive constant
so suppose $G(u,v) = F(u,v) - B$ is linear for some $B$
Rafilouyear2026
Then, since we're in finite dimension, $G(u,v) = A\begin{pmatrix}u\v\end{pmatrix}$ for some matrix $A$
Rafilouyear2026
so are we writing the parallelogram D as some linear combination of vectors?
and B is some corner we pick
D is the image of R through an affine transformation
so each vector/point in the square R is mapped to some vector/point in D
so can we construct the parallelogram using F(u.v) = <x,y> = <x0,y0> + u <a0,a1> + v <b0,b1>
and i can pick x0 and y0 as a corner on the parallelogram
say 1,1
yep
and u and v vectors are direction vectors im assuming
oh
but if you make "u" bigger, meaning moving horizontally in the positive direction,
that corresponds to moving in direction (a0,a1) by that much on the parallelogram
and changing v, meaning moving vertically when you're on the square
corresponds to moving in direction (b0,b1) on the parallelogram
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>
these are the edges of the parallelogram?
so F(u,v) = (1,1) + u(1,-1) + v(1,2) maybe
you should rather say "from (1,1) to (2,0)" since you're starting from (1,1) which is your base point
ohh yeah
and you get <1,-1>
same thing for b0,b1, even though you got the correct vector
and i expand this and i get the stuff underlined in red
exactly the F they got
ohhh okayyy thank you 🙂
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
ohh so like we could have picked (2,0) to be x0,y0
and if you had kept x0,x1 to be (1,1), and flipped the directions to (a0,a1) = (1,2), (b0,b1) = (1,-1)
you'll make a minus appear in the determinant, because you inverted the orientation of the space
and we can just take the absolute value of this right
if F doesnt preserve the mapping orientation
yea like |JF(u,v)|
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
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
my body is reacting viscerally to that, but ok
lmao is this not normal notation i js started this topic so idk
The determinant of a matrix A (no absolute values) can also be written as |A| in linear alg
ya
then how would you indicate absolute value if u cannot use | | signs
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
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
Rafilouyear2026
oh idk then Lol
It's not your fault
your book just decided to go wild on notations and go against all notation conventions generally accepted
lmao
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
ok so $\frac{\partial(x,y)}{\partial(u,v)}$ is also the jacobian determinant and they also call it "Jacobian"
Rafilouyear2026
lmao idk 
Ok so there are indeed some sources which prefer using "Jacobian" for "Jacobian determinant" instead of "Jacobian matrix", but they're in the minority
But the notation JF(...) for the determinant is still wild to me
hello higher!

interchanging notations between a matrix and its determinant is the scariest thing ever
maybe i will continue using it then to be different 
anyways thanks for the help!
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how would i type this into a ti 30 calculator
i keep getting the wrong exponent
its supposed to be like 2.60x10^-9
but i keep getting like 2.60x10^-49
did you type 10^17
no i typed 10^-17
hmm
,w calculate ((6.626*10^(-34))(3(10^8)))/(7.65(10^-17))
just spam brackets
Hi
Wsg
Thank youuuu
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Where am I going wrong here?
I should be getting $-\frac{x}{12y^{14}}$
Vortac
$-\frac{3}{36}=-\frac{1}{12}$
JamR_71111
just needed to reduce the fraction a bit, everything else is good
but $x^{4-4} = $x^0$?
Vortac
Compile Error! Click the
reaction for more information.
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I'm ngl I thought that was an 8 cause it was so zoomed out lol
Ohh okay
I need to remember to always reduce
yeah, easy to forget sometimes
I was thinking somehow 36 needed to be in numerator and 3 in divisor
thanks everyone!
Np 😁
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Can i get some help on this?
@timid dune Has your question been resolved?
@timid dune Has your question been resolved?
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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
@opal wolf Has your question been resolved?

What's |xn| here?
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If every closed and bounded subset of a metric space $M$ is compact, does it follow that $M$ is complete?
Kasli, the Scourge
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.
@random sky Has your question been resolved?
Kasli, the Scourge
Compile Error! Click the
reaction for more information.
(You may edit your message to recompile.)
Kasli, the Scourge
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.
i think it's good modulo typos
your TeX is good
It should be k > N
But anyways its fine
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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Just one last thing: the closure of an open ball isnot always identical to the closed ball d(x,y) <= r. I mean ir is fine here but I feel ike relying on the definition of the closed ball is more appropriate than using thr closure of the open ball
did they assume this? i mean they showed cl(B_r(x)) ⊆ B_(r+1)(x)
well they didn't but it's true
'...\overline{B_r(x)} is therefore a closed set containing S'
Yeah its fine
You raise a good point. I will amend that part of the proof.
Thank you.
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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Find the number of integer solutions, where each 0 <= x_i <= 10
x_1 + x_2 + x_3 +x_4 = 21
Let A_k, 1≤k≤4, be the number of solutions where x_k > 11, ...
so we have to find S - (A_1 and A_2 and A_3 and A_4)
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|?
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?
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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
My mind is kinda fried rn, I can't think of anything
we weigh 2 at a time?
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)
wdym 2 at a time?
two cubes at a time on balance scale
any amount is fine
Don't have to be one each side
ok'
hmm
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.
I get the upper bound
I'm still trying to understand this one
There are 14C7 possible distriubtions of the aluminium and steel balls. There are 3 possible results of each weighing. That means that after n weighings, there'll be 3^n possible states. You need to map injectively from those states to distributions to decide, that means 3^n >= 14C7 meaning n >= 8
A balance puzzle or weighing puzzle is a logic puzzle about balancing items—often coins—to determine which one has different weight than the rest, by using balance scales a limited number of times.
The solution to the most common puzzle variants is summarized in the following table:
For example, in detecting a dissimilar coin in three weigh...
it actually has a wikipedia
The distributions thing is messing up my mind
Imma give it a look
Okay those are kinda high level stuffs
to me
it seems like a pretty difficult problem
It's supposed to be a uni entrance exam problem, shouldn't be that hard
I get the idea of mapping but why are there 14C7 distributions
You have to pick 7 aluminium balls out of the 14, the rest will be steel
Can u use computer to solve it? And do they require a proof of optimality or is it like a logic thing where you just come up with some good solution that works?
do they perhaps have a file with intended solution somewhere
no, I'm allowed to use calculator
But computer is not
Yeah they will give me the solution today or the next sunday, I'm not sure
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
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 🥀
,calc 100C19
The following error occured while calculating:
Error: Undefined symbol C19
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
I thought of that, but is that possible
Yes
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, ...
ohh
Yes I just thought of tha
thats a famous way to color
It seems possible
i think so
the question is whether its optimal
hm
it feels like it both should and shouldnt be
try add one more color to that, and see if we can get one more "lonely" cell
idk doing these problems fried my head 
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
i think this is kind of sudoku
Oh wao you're right
How come I didn't think of that
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
True I meant
19*100
I instantly thought this's wrong
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
It's a national one
Ha Noi University Of Science and Technology
WOWZ
I meant it's the top 1 engineering uni
Oh, interesting
In my country
that kinda question is high school entrance test here
I wonder if this can be solved simply as well
letsgo
good to know i wouldnt manage to get to HS in ur country
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)?
My answer is 1/2 
Just want a clarify
wait im doing it
There'll always be exactly one person sitting at the wrong seat, unless someone sits back to alice's seat and closes the cycle
so we just need to find the probability that someone sits back to alice's seat
ig i phrased this quite imprecisely, its not what i meant
i cant even find a way to phrase it correctly
I think the last person the choose a wrong seat have to choose either Alice's seat or the 100th seat
If the wrong seat is still there though, which is where you went wrong i believe
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
Lmao where do you live
it depends wether the 99th person chooses the right seat or the wrong seat
korea
it's not
Ic
wait hmmmm
Wait north/south
nein
oh i kinda get the 1/2
I don't know how to phrase my argument correctly 🥀
Which korea is left
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
south lol
and that probability must therefore be 50%
oh yeah makes sense
its basically because from the perspective of passengers, alice's and last guy's seat are indistinguishable
By symmetry is not so satisfying to me tbh
But my answer is correct so
yay
yay
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
If alice sat on nth person's seat everyone below n will sit on their seat, but from nth person if chooses to sit on Alices seat then everyone else can sit on designated seats, else nth person sits on kth seat, continues till last person
Oh that's same argument as mine
Kinda like this
I'm slow typing missed all you guys said bare me if if i repeat
Lesgo
?
I doubt
It's similar, but i dont think its necessarily the last person. If the 3rd person chooses alice's seat, then it's already determined that the 100th guy will sit at his seat. If 3rd person chooses 100th guy's seat, it's already determined that he'll sit somewhere else
you escaped from north
say in dm
If the third person choose alice's seat then 4rd-99th passenger would sit at their designated seat
So he is the last person to choose a wrong seat
but what if 3rd guy chooses 100th guys seat?
beside the 100th guy
I meant

still in between them everyone would be in their seat
yay
so the only unsolved question is the metal cubes one, which I have no idea how to do
i know how to slove if theres one but 7 makes me dizzy
you can try that later, I still have some weird questions to do
okay doin hw here is kinda fun, 1 more ig
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
this would be different if a person can choose seat irrespective if their original seat is available
0 is obvious
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
wowx
yeah I get that
x + y + z, x + y + w is an integer would require z - w to be an integer
HMMM
wait what
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
yeah thats
z-w = 1 or -1 or 0
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)
oh interesting
NICE
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
Haven't thought of this
Wao that's impressive
me too
You probably dont even need all this casework
i was just expecting the distinct case to be difficult
so i did the casework first, but it probably wasnt needed
you shouldn't
but thats the logical way
We have 40 questions in the span of 60 min
wow
crazy
damn
1.5 min/
we have 7questions per 100min
whats the avg score?
i can imagine spending 5 days on the og question and still not getting it
lol
Hmm, so there're Math-Science-Comprehending session
Average is 50-60/100
like 75% taker
and are most of the questions easier than the ones u sent?
this feels vietnamese
lol he is
yeah, there're around 5-6 hard one
others are mid
Not so easy tho
our test too
they are all hard
the time is what make it hard here
avg asian shit 😢
And the precision
also a lot of creativity to solve
you know my questions
your are olym type 15-30 min/q
We still have some hard geo questions 
yeah ill send you the pdf
what is your exam called again @charred carbon
the TSA
imma check some past year papers if available
I don't think there's any in english
fine, i'll just have look
btw are all q solved
for this channel
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 .....................
hmm thats interesting
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
That's interesting
Oh that's not many problems I see
but it needs a lot of creativity and fast thinking
ill be back in 15 minutes
here perpendicular to centre of intersection cicrle radii of sphere and radii of circle form a right triangle

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
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gn
i suck at 3d
but distance b/w center of S1 and S2 = R1 + R2
now we relate that to AB
idk how
you're not alone mate
AB is r^2
✅ Original question: #help-28 message
Here letmme show you something
🫂
what do i need to get
this is for ?
i love geometry
Just an example kek
oh
for how 3d geo here look like
😢
we don't learn 3d for same reason
Oh that's not a hard problem
, the hard one has even more lines than that
My nightmare back when I prepared for comp
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its beautiful
i have seen this
yeah
i dont remember
I'm fionna
who fionna
ohk
right

ouch
wait fionna is girl
do you have multiple personalities or something (light joke)
lol
what
wut
lance type shit
Not lance lmaooo
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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
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
@opal wolf Has your question been resolved?
It was just the idea of proof
I have to make it formal
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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.
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 :<
maybe we could try rephrasing it all in terms of mod
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
agh got sniped 
-#
Celine is backkk
um ok let me do it :3 thanks
ooki have proven the first one, the second one seems a bit complicated and general? is there a way to show how many positive divisors of n 😅
do you mean that you proved mod 3 or that u proved the d^2 - 1 question?
the d^2-1 one
hint, prove that for every divisor d of n, d+(n/d) is divisible by 24
and hint #2: Q1 can be useful
but how can that be helpful in any ways? i dont really understand why is ther suddenly d+(n/d), it looks quite random to me 😵
you can pair the divisors into pairs d, n/d
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
sure, ill come back in a minute
oh i get it now and also i solved the problem, thanks alot :3
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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.
I don't have a phone rn
so you swear your life on the fact that it says "she fell short" and not "she fell Rs. 999 short"...
yeah, i was going to say you should skip.
please don't call Ann 'bro'
please edit the word "bro" out of this message.
he has left...
he's not left the server as far as i can see
i mean offline
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they left chat without closing it lmao
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I didn't ignore you I really went offline. Why would I do that. Sry if it made u feel uncomfortable 😔
thank you for editing it out as i asked.
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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
i do exactly this with the bouncing and everything
ok thanks
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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
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
Hypothesis....
xn≤yn≤zn
So lim(xn)≤lim(yn)≤lim(zn)
Doesn't it help
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
I didn't understand how this can't tackle this case
you are writing lim(yn)
how do you know that you can write that
how do you know that it is a number
am i correct
Ok thank u
What changes in this proof , to make it fully correct
they show -eps < y_n-w < eps
which essentially directly shows that y_n converges to w
which is immediately what you want
So essentially I have to follow book method only
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
I was just trying to find an alternate proof
Seems like , it diverts to books prove ,...only
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not able to start
Bayes‘ theorem?
classic bayes
yup bayes
cn u do without that
yeah but you wd js find the formula itself again
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Pls explain 2nd degree 😭
u mean ax^2+bx+c ?
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3x^2 - 7x - 5 = 2x^2 + 4x - 9 like ts
