#help-49
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yeah the other options are way wrong, there's no real choice
definitely correct
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why do we need to take a determinant for evaluating eigen values?
Eigenvalues/eigenvectors are such that $A\vec{v} = \lambda \vec{v}$. Rearranging, this means that $$A\vec{v} - \lambda I \vec{v} = \vec{0}.$$
And factoring we get $$(A-\lambda I)\vec{v} = \vec{0}.$$
Azyrashacorki
yes
Notice that since we usually want to find nonzero eigenvectors, this means that the matrix given by $A - \lambda I$ is not injective, so if there is such a pair of e-value/vector, the determinant of this matrix has to be 0.
Azyrashacorki
why
What part is not clear?
why determinant
A - lambda I is not injective, meaning its kernel is not just {0}.
When the determinant of a matrix is 0, it means specifically that the kernel it not just {0}.
kernel?
The set of vectors that get sent to 0 is called the kernel
i.e. if Ax = 0, then x is in the kernel. If the matrix is injective, the only such vector is 0
@last slate Has your question been resolved?
Well you might recall that a matrix is invertible iff its determinant is nonzero.
And it's invertible iff it's bijective, and thus injective, which means its kernel must be just {0}.
So if in our equation we know the matrix can't be invertible, its determinant has to be 0.
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$\int_{-1}^{1} \int_{x^2}^{2-x^2} f(x, y) , dy , dx$
Should the reversed order for this integral be:
$\int_{0}^{1} \int_{\sqrt{y}}^{\sqrt{2 - y}} f(x, y) , dx , dy$
Calc III Victim
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ABCD is a rectangle, E is an extension of CD and F is the point where BC and AE intersect, if AB=18, BC=24, area of CEF is 144 greater than ABF, find BF
let BF=x
we can see ABF≈CEF, let BF/FC=y
BF×18/2=BF×y×18×y/2-144
9x=9xy^2-144
x=xy^2-16
BF+CF=24
x+xy=24
idk how to solve this system of eqyatuon
@viral dagger Has your question been resolved?
@viral dagger Has your question been resolved?
@viral dagger Has your question been resolved?
i solved it without resulting in a system of eqs
using the intercept theorem, i found a relation between BF and CE
and then using the provided info that A(CEF)=145 A(ABF), i found another different relation between BF and CE
i got CE expressed in terms of BF from one of the two relations, and i substituted it in the other relation
i ended up with a quadratic equation in BF etc...
hey, how did you get A(CEF)=145 A(ABF)?
Area of CEF is 144 times greater than the area of ABF
its not 144 times greater
CEF = ABF + 144 ABF
fair enough, probably couldve worded that better
I interpreted it as 144 times..
but but but, im pretty sure it's like I said
The quadratic I got was like
beautiful, you know what i mean
3x²+x-12=0
if I didn't treat it like that, idk what woulda happened
there's 2, one from the intercept theorem and one from the areas relation
which one do you not see!
both :3
Emily
smort
use double $$
Emily
thats what i wanted to say :p
:p
and the other..
its simple, you can figure it out
find area of ABF, find area of CEF
set CEF = 145 ABF
you get in the end, after expressing CE in terms of BF:
$CE = \frac{2610 , BF}{24-BF}$
Emily
$\frac{CE\cdot (24-BF)}{2}=9BF+144$
Skill_Issue
and when you sub, it works out nicely
but thats wrong :<
$CE=\frac{18BF+288}{24-BF}$
Skill_Issue
you can do it!
yeah yeah, i guess it works
whether you interpret it as i did, or as you said 144+ABF
nice!
Skill_Issue
hahaha
okay wait ill try it myself
ans chouces are 9 10 12 16 none of the above iirc
its 9
its correct, you surely made a little mistake, thats all i think 😛
do it again slowly
one of the steps: 18 BF² + 288 BF = 18 ( BF² -48 BF + 576)
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i read a point about skew symmetric matrix, that all diagonal elements must be zero. whys that
what's the definition of a skew symmetric matrix? start with that
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can anyone tell me what's wrong here?
( i got 0/7 because it should be a disproof)
well I mean even if its not unitary it could still be < at that point
which would make the whole thing <=
so ur saying it's only the first direction?
yes
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hello
im really confused on how we can interchange taylor series at a point with mclaurin series of shifted functions
is there more justification than this
it was me but it wasn't really relevant
g(x) = f(x+a) is not enough?
since g(0) = f(a)
It's just odd that you were introduced McLaurin series before Taylor series
not that odd, was for me too
makes more sense tbh
A horizontal translation just corresponds to another function, if it still converges centered at 0 you can use the McLaurin expansion
i don't see how this relates to f(x). Like the main issue is that with the above statement is that i can't see (or maybe understand) the algebraic statement that links f(x) centred at a to f(x+a) centred at 0
you can retrieve f(x), by the fact that g(x-a) = f(x)
what do you mean by that?
sorry i was talking about finding the taylor expansion of f(x) centred at point a
okay so the above statement is saying that we can instead find the mclaurin series of f(x+a)
indeed
would i be able to prove using the expansion (i.e. f^(k)/factorial(k) * (x-a)^k) that f(x) at point a= f(x+a) at point 0
i know my notation is not very good, but hopefully the essence of my query was conveyed
@glad merlin Has your question been resolved?
im not sure i understand you, but yeah
just set g(x) = f(x+a)
apply maclaurin expansion
then you can get f(x) back by doing g(x-a)
@glad merlin Has your question been resolved?
@glad merlin ?
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im very confused as to how to graphically represent the additions/subtractions of vectors
suppose that i want to go from A to C
but if i write a + b it is wrong as the 2 vectors are in opposite direction
when im asked to write vector AC
it means that how would i go if i want to travel from A to C
go throuhg B right?
are you saying distance vs displacement
sorry i forgot to phrase my concern. my concern is the tip to tail method
like you apparently need to
rearrange the vectors?
ye thats what im demonstrating
and draw a new vector from the tail of the first vector to the tip of the second vector
yup
i always visualize it by flipping the second vector
and adding
nope
ahhh
$\vec{AB}=\vec{B}-\vec{A}$
Hamdy Hisham
how do you prove that (ive heard of it before tho)
thats only if A and B are position vectors right
meaning they face different directions
like they spread apart
like this
well right from your drawing $\vec{AB}+\vec{A}=\vec{B}$
Hamdy Hisham
that isn't necessary it can work at any vector
why is it ab + b
look at your drawing and you will see
i dont follow
how is that possible
.
nope
if that were true
that means that AB=-A-B
yeah but like
based on the direction of the vector
to represent vector B in terms of vector A and vector AB
wait whats the '-' and '+' signs based on
note that $\vec{AB}$ is defined as the vector from the tip of $\vec{A}$ to the tip of $\vec{B}$
Hamdy Hisham
yes
the minus sign changes direction but with the same magnitude
ye so like for this one to express vector B, following the blue arrows, vector A + vector AB (a to b) = -vector B because the blue arrow is the opposite direction to the actua vector?
nope
when adding vectors
the resulting vector is from the tail of the first to the tip of the last
so it will be +B
so the tail of the first meaning
the tail of A?
am i overcomplicating
is just remembering what vector AB enough
yup
yup
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How do i do the 1st question
Isole y
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hello
does anyone know what i have done wrong for the first part
as in the matrix representation of A2
i thought it would just be the matrix that was given to us
but apparently not
okay figured it out
basis means it must be linearly independent
not all of the vectors in the matrix are linearly independent
so ig i just take the ones that are
thanks
actually i'll keep this channel open just for a little bit more time
i may have another question real soon
@glad merlin Has your question been resolved?
how would you go about determining the radius of convergence here
i know its by the ratio test, but im getting a weird limit
which is approaching inf
that doesn't seem correct
What did you try?
the (1+1/n)^n is the e identity
but the (1+n) part makes it diverge to inf
i thought perhaps the radius was inf
but its no bueno
Do you recall the theorems related to power series and radius of convergence?
In any case, you used the ratio test and (correctly as far as i can see) the limit is greater than 1 (for most x)
what can you conclude in such case?
oh it diverges
converges
In any case the radius of convergence should be 0 if i havent missed anything
meaning that R = 0
yup!
What was your work here?
i got this weird abs((x-4)/(n+1)^2)
and 1/(n+1)^2 also diverges to Inf
except when x = 4
so i thought that R will also be 0 in this case
but once again no bueno
have i perhaps done my factorials incorrectly
So the reason i mentioned theorems related to power series is because you can mostly ignore the (x-4)^n bit, as the coefficients here is what matters
i.e try the ratio test soley on the coffiencts c_n = 1/(n!)^2
if that is easier
ok i will try that again
just want to confirm one thing tho, ((n+1)!)^2 can be expressed as (n+1)! * (n+1)! right
sure, just depends on what youre doing i suppose, in our case we just "factor" the power 2 out of the quotient
if that is easier
i.e it feels reasonable to rather do a^2/b^2 = (a/b)^2 in this case, no?
so as n approaches infinity?
yeah
oh i didnt catch that
it goes to Inf
Are you sure?
bro
just plug some really big number
no youre not dumb, again; all the right calcualtions but just a little mistake id do aswell
So this is the ratio test remember?
and in this case it seems this happens no matter what x we have
What is inf?
as in like plus or minus infinity
or like the radius of convergence is the entire number line
Yea
Also, im not sure if youve seen it, but one theorem says that this limit you find, in this case 0—well the radius of convergence is just the reciprocal of that, and we're being a bit lazy here and letting 1/0 = inf (but this is not what happens in the proof of course)
yup and it is, and so its undefined
but in this rare instance its just a standard convention and theres theory to support the shortcut here, its just a way of symbolically expressing it; but its not the same as litrelaly doing 1/0
makes sense
also in regards to that previous question, sorry for just randomly ditching it
should have told you before
well usally its preferable if you use a channel per question
i was talking the mclaurin one
it was just confusing is all
Oh i see
Well i dont mind, i only help because i like to
np!
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I did the first part pretty easily, but I can't seem to get anywhere with the second part.
we can write expression for f(1) f(0) and f(-1)
but Im not sure how that really gets anywhere
f(-1) |a-b+c| <=1
f(0) |c| <= 1
f(1) |a+b+c| <=1
use the expression for f'(x) you just obtained
yeah I was thinking I could try to find the maximum of it
overthinking
but the negative at the end makes it a little trickier doesnt it?
how big can the first term be at most
3/2
what about the next one
surely 3/2 as well
well I should be slightly more precise. how big in absolute value
uh wait made a mistake in my head. not as nice as I thought
ok, so obviously if the first one is 3/2, then the second one couldnt be 3/2 aswell
so we have to make that more precise
yea
thankfully we can get rid of the f(1) and f(-1) by bounding them from above
so we are just left with |x+1/2| + |x-1/2|
we want to bound |f(1)(x+1/2)+f(-1)(x-1/2)-2f(0)x|, yes?
yes
by triangle inequality, thats <= |f(1)| |x+1/2| + |f(-1)| |x-1/2| + 2 |f(0)| |x|
sorry whats triangle inequality?
|x+y| <= |x| + |y|
most useful inequality ever
abs of sum is annoying. sum of abs is better
alr
well if x = 1 then the max value of
f(1)(x+1/2) is 3/2
f(-1)(x-1/2) is 1/2
-2f(0)x is 2
and 3/2 + 1/2 + 2 = 4 which works
idk if it holds for -1
well but what about all the other values of x
keep working from here
and then, like you wanted to at the beginning, find the max of this
um, the max of that would be 3/2 + 1/2 or |-1/2 - 3/2| so 2 in both cases
well cause x is either -1 or 1
why cant x be 0.71396
hmm if a were positive, the max would have to be at either -1 or 1
but im not too sure tbh
make cases depending on whether x+-1/2 is positive or negative
so if x+1/2 is positive then the max is obviously x = 1
and if x + 1/2 is negative then the absolute max at x = -1
if x - 1/2 is positive then x = 1 is max
if x - 1/2 is negative then x = -1 is max
oh yes so if we take the max value lets say 2
then max of 2 - 2f(0)x, when x is 1
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Ok so what is the area of a hémisphère?
around 461?
I meant the formula
yea
And what about the cône ?
what would the radius of th cone be?
The same as hemisphere
How ?
idk
Wait we ll have a problem
Nah it will be 1/2* total surface area of a sphere + pi*r times sides length
Should be 637 or 638
Counting the base, which is not part of area surface
All good
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can someone explain how to do this?
im used to making a matrix and solving
but i cant do it here
all i have right now is $0 = asin^2(x)+bcos^2(x)$
not sure how to make a matrix out of that
for a function to be in $\text{span}{f(x),g(x)}$, they must be a linear combination, so $c_1f(x)+c_2g(x)$
yes
fish
wait you already had that step
yes
since $c_1, c_2 \in \mathbb R$, i assume you can set both to 0 to confirm the first one
fish
i think so, not really sure how to set up a matrix for it
ok that makes sense
otherwise, it would just be the product of 2 vectors, which gives about the same information as writing it as an equation
what about for the sin(x) + cos(x)
i wrote
$sin(x) + cos(x) = asin^2(x) + bcos^2(x)$
how did u find that value?
wait nevermind
i guess i could say $a=1/sin(x)$ and $b=1/cos(x)$, but idk if that works
fish
yeah, assumed so
this is essentially an infinite system of equations, you plug in a value of x, you get a linear equation that a and b must satisfy
if you can extract a set of inconsistent equations for a and b (by plugging x's that simplify nicely with the sin's and cos's for example), it shows that sin(x) + cos(x) can't be in the span
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hey, im just curious, how many times does 2^n and 3^m get 1 away from eachother?
i can count for small numbers, (2,3) and (8,9), but is there any more?
those are the only instances
in fact, a^b - c^d = 1 for a,b,c,d>1 has at most 1 solution for given a,b (unless a,b=2,3)
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ye should be a paper due to leveque
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my progress right now:
n=2 left, all other cases solved
anyone offer me some help to finish it off?
@granite smelt Has your question been resolved?
i see
Firstly odd numbers are trivial, then just n>2 even numbers can be taken care ofby LTE
The reason why LTE doesnt fucking work for n=2 is cuz it’s p|n-1
No prime factors
And now i’m malding qwq
Some more elaboration:
Basically i just assumed what’s left was 1
Like
n^k = 1 (mod k)
In other words k|n^k-1
Then now if p|n-1, then n-1|n^k-1, so if i pet k=n-1, it’ll work:
n-1|n^(n-1)-1
To show that there are infinitely many cases, take k=(n-1)^m
yeah i get you
Dont directly spoil if you do get it tho
ye
@granite smelt Has your question been resolved?
ok bruh i think i got it
somewhat convoluted though
take k=2^m * p
been too long, im out of shape
rip
yikes
what motivated this
stupid problem
(its 2014 N4
flt thing
yeah basically my solution
i woudlve mever found sol 1
sol 2 looks more approachable
dude what is this
💀💀💀💀
not in a million years is that going on my mind
i wonder what the motivation behind 3*4^i is
yeah that seems pretty silly
3 probably comes from the fact that the remainder of powers of 2 mod 3 form a seq with period 3
well that goes for pretty much all primes
maybe, i think it's just that 2^n*p is a solution frequently
for n large enough
the flt thing is kind of cool, so im glad i found that
you just go 2^k equiv 2^(2^m)
so you want to find p so that 2^(2^m-m)/p is odd
that's kind of insane
for many of these kinds of problems its safe to assume that flt is hiding somewhere
Usually
if there is an exponent
and there is a mos
mod
😱😱😱😱😱fermat: ever heard of prnis theorem
i mean little theorem
anyway cool
@merry pewter have you been to the imo?
not a lot of people can take on N4s easily
no
not easy
lucky because i am best at this kind of problem
ye
@granite smelt Has your question been resolved?
well
it's a far goal
i'd like to if i could make it onto team
dude but
jesus cheist
bro solves n4s under an hour
damn
are you still under 18
well i am in college, im 19
yeah
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I think using Vieta's formula is an approach to this problem
uh like sum of roots = -b/a
and product of roots = c/a
so i take write both and simultaneous equation?
Just apply Vieta's formula separately in those and try to eliminate the roots and m
ok ty i try
Would this work
yeah I see no error
np
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Why is a basis for A also a basis for B = (A)(A^t) ?
the original problem was: construct a matrix with (1,0,1) and (1,2,0) as a basis for its row space and its column space.
they answered it by multiplying $\begin{bmatrix}
1 & 1\
0 & 2\
1 & 0
\end{bmatrix}
\begin{bmatrix}
1 & 0 & 1\
1 & 2 & 0
\end{bmatrix}
$
to get $\begin{bmatrix}
2 & 2 & 1\
2 & 4 & 0\
1& 0 & 1
\end{bmatrix}
$
johnseymour20
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I am just asking why the continuity correction is addition rather then subtraction here
for p(z <= 2.2) I understand it would be addition because it has to "wrap around" the 2.2 value inclusive
but for what is p(z < 1.5) shouldnt it be that after the correction it is p(z < 1.5 - (0.5/50))
where 50 is the sample size?
Please ping me if you know the answer, thanks.
@icy cypress Has your question been resolved?
<@&286206848099549185>
but P(1.5 < X <= 2.2) is not equal to P(X<=2.2) - P(X**<**1.5)
it's actually: P(1.5 < X <= 2.2) = P(X<=2.2) - P(X <= 1.5)
before
Notice the X range, 1.5 < X <= 2.2
using your thinking, which is
P(X<=2.2) - P(X<1.5)
X can be 1.5 right?
😛
But it mustn't
it's an interval thing, that's all
so you just have to look at this questions and like think big brain style
while doing it
exactly
X must be strictly greater than 1.5
can't be exact 1.5
if you use < 1.5
because when p(x<=2.2) it has to be every value below
i didnt understaaand
or is it like the weird thing where when u sign flip with the normal distribution coz the alligator (<>) is the wrong way for your z table?
does it turn from > to <= because of the x <= 2.2 or because of the ^^
i dont know how you are thinking about it 😛
hm
the way im thinking about it is...
neither
in order to have the required interval ]1.5 ; 2.2]
you grab ]-oo ; 2.2]
and you remove from it
]-oo ; 1.5]
1.5] not 1.5[
sorry, I am not familiar with this notation
it's okay, it's okayy
so its nothing to do with the normal distribution
yesss, just the interval
you could just consider the equality 15 < x <= 22?
im not really familiar with like messin round with equalities like that
maybe I should be
are we on the same page, or not yet 😛
HMMMM WAIT ITS LIKE SAYING THE X>15 AND THEN THE PLUS X<=22
THEN IT FLIPS VIA NORMAL STUFF TO X<= 15
about why it is this: P(1.5 < X <= 2.2) = P(X<=2.2) - P(X <= 1.5)
and not this: P(1.5 < X <= 2.2) = P(X<=2.2) - P(X < 1.5)
uhm yeah coz the 2.2 is like above it
and for x<=2.2 x must also be <= 1.5
yes, 2.2 and less than 2.2 are all included
not just skip the 1.5 value
kk thx
good old paint app
no worries if u dont wanna'
its okayy
👍
yes
notice it includes 22
mhm
yeah
okay, so far so good
yes
to have this interval, we start with this interval, i will draw it in purple, wait
and we remove from it this one
in orange
my marker bout to have the full rainbow
thats how you get the needed green interval
WTH
😂
what you were doing is... you were grabbing the purple interval
uhuh
and you were removing the orange interval, yes
take big daddy purple and remove small orange
but without removing 15 too
to get green
which is the required
and as you see, orange is P(X<=15) and not P(X<15)
i know you understood, just wanted to make it clear
Hahahhahaha
can i like rep u
you are welcome
<@&286206848099549185> make dis person helper
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how what why
have y tried to simplify it
i moved -1/tx-4 to the right
now i have 36x^2+16x-20/tx-4
should i multiply tx-4 by -9x+5
?
Factor numerator 36x^2+16x-20
You can factor better
how
Factor inside the parenthesis
u mean solve the quadratic equation?
Factor the polynomial 9x^2+4x-5
Factor like (ax-c)(bx+d)
(9x-5)(1x+1)
So u have 4(9x-5)(x+1)
right
-9x^2t+36
?
Where did you get this
-9x+5
-4(9x-5)(x+1)
with the other 9x-5
Looks good
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if ϕ : V -> R^n is a linear isomorphism, then does V=R^n?
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i have no idea what to do
ahhh wait
is it that cause its a to the power of 3, technically the most amount of intersections is 3?
so d jusy equals k
but then its a bit misleading to put 4 times if its infinite intersections
yeah the wording is weird
but it doesn't say exactly 4 so i guess it's fine (?)
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A prism whose base is a 9 -sided polygon is intersected by a plane. In which case could the cross section be a 10 -sided polygon?
A. when the plane is parallel to the base of the prism
B. when the plane is perpendicular to the base and contains two edges of the prism
C. when the plane is perpendicular to the base and contains one edge of the prism
D. when the plane makes a narrow angle with the base
@inner timber Has your question been resolved?
I don't think it is A because then it would be a 9 sided polygon since it's parallel to the base
I have no idea bout the other 3 however
@stiff heart
@inner timber Has your question been resolved?
@inner timber Has your question been resolved?
B and C would make a rectangle
D can make a bunch of stuff, but the idea is that if you just clip one angle, then you will add an aditional side there
should i model it?
yes pls
@inner timber Has your question been resolved?
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I don't understand what this question wants from me. I am in Calculus II and this question is part of a Taylor Series assignment. I am sure there is supposed to be a way to use my knowledge of Taylor Series to solve this question, but I am not seeing it.
Do you know binomial theorem
i'll look into it one sec
oh yeah ive seen that
alright thanks @fossil knot
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✅
But you can see that when you expand the expression, its degree is 144 with leading term x^144
Binomial theorem tells you there's nothing with x^143
Why is that
Wouldn't there be one
Nevermind i see
Wait I think I see now
But once I expand it, won't there the leading term be (x^8)^18
cause its (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) * (x^8) etc. til its 18 times
np
(so close, you copied it 17 times)
Oh i didnt think i was even close to 18 times
I just said etc
so basically
the factorial of the leading thing's exponent
so 144!
what
cause every other term below 144 would go to 0
by the 143rd derivative
and it would be 144 * 143 * 142 etc... all the way down to 1?
Or am i wrong
i just dont get why x going to 0 would be there either
maybe im lost
Each time you take the derivative of $ax^n$, the exponent decreases by 1
Ari
So
Would it go all the way down to
(144)!x
but then its x=0 so isnt it 0
i have 2 tries left so idk if i should check
exactly
omg
THank you
But no way it was just a GIMMICK QUESTION
thank u for ur help tho
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lol
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I have no idea where to start
if i add them why does a common factor appear?
