#help-38
1 messages · Page 135 of 1
,w characteristic polynomial of ((2,0,1);(2,1,3);(1,-1,0))
i got the same
,w factorise -x^3 +3x^2-4x + 3
bruh
Try this
i dont get it
yea but
i just dont wanna do it the normal way
we're meant to solve this question using characteristic equation
@wraith hinge Has your question been resolved?
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@wraith hinge Has your question been resolved?
Huh, this is weird. It doesn't seem like the polynomial f has much to do with the characteristic polynomial of A.
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,tex \begin{enumerate}
\item Find all values of $a > 0$ such that the area enclosed by the line $y = a$ and the graph of $f(x) = 4x^3 + 12ax^2 + a$ is equal to $3a^2$.
\end{enumerate}
938c2cc0dcc05f2b68c4287040cfcf71
!status
What step are you on?
1. I don't know where to begin.
2. I have begun but got stuck midway.
3. I got an answer but I was told that it's wrong.
4. I got an answer and would like my work checked.
5. I have a question about someone else's work/solution.
6. I have completed the problem and don't need help anymore. Thank you.
7. None of the above
1
just roleplay as someone finding the area as enclosed
938c2cc0dcc05f2b68c4287040cfcf71
this is wack, frankly
how do you normally find the area of a region enclosed by curves?
mmm
its one definite integral minus the other definite integrals
problem is I dont have a range for the boundaries of the integral
@urban copper Has your question been resolved?
im not sure
@urban copper Has your question been resolved?
First consider drawing a crude sketch of the curves in a graph
Maybe given certain values of a
Then what you want to find is the area enclosed, and as such the intersection between the curves (which are reasonable)
okay
how to do it
I dont have access to calculator only pen and pencil
@sudden mist
Yeah so how would you usually sketch a curve in a graph?
When you say never? Do you mean you’ve never even plotted points before? As that might be a start
But say, y=a
This you can surely sketch given a reasonable value for a?
im newbie
Why are you dealing with integrals when you’ve not even dealt with such things before?
well y=a is a line that passes through a but idk
Or for that matter the undergrad role assigned?!
Hm, okay let’s try doing this without a graph
It will be a very big handicap for us
But let’s try I suppose
sure
So we can straight off try and find the intersections for both curves
How would you do that?
equate them
Right! Try and do so and show what you arrive at
?
What?
now what
Solve for x
938c2cc0dcc05f2b68c4287040cfcf71
I’m confused, how are you able to do this step and not see what to do next?
Well maybe I’m being harsh, but it’s not at all clear what you need help with here if you’re not willing to explain
?
This was meant towards your deleted wolfram message
,align &\implies 4x^3 + 12ax^2 + a = a \ &\iff x^2(12a + 4x) = 0 \ &\iff \begin{cases} x_1 = 0 \ x_2 : 12a + 4x = 0 \implies x = -3a \end{cases}
938c2cc0dcc05f2b68c4287040cfcf71
Okay, so now that we know the intersections for the x-coordinate we can try to continue
Since a>0
We can make the reasonable assumption that the enclosed area must be wherever x is non-positive, since the question states there is one and we’ve just seen where these curves intersects
We can try to show this more carefully but I think it’s clear for our purposes
We haven’t shown it yet, so I’m wording it like that
Furthermore, we can show that the curve y=a will be under the other curve
Which part?
how
Furthermore, we can show that the curve y=a will be under the other curve
Yeah that’s what I was about to ask
Can you see how?
(Try and do it)
I’m not here to spoon feed you all the answers
Okay so we want to show that 4x^3 + 12ax^2 + a > a for x in between -3a and 0
sure
?
In other words, 4x^3 + 12ax^2 > 0
Try and see if this true for the given x
And remember a>0
?
Oh Ok so you’re just trolling?
Lmao why’d you delete the message?
I think Im done helping here.
you here?
ye
you first need to find out where y = 4x^3 + 12ax^2 + a and y = a intersect
do you know how to do that
you mean $4x^3+12ax^2+a=a$
@urban copper are you stuck
yes
try factoring
938c2cc0dcc05f2b68c4287040cfcf71
,, 4x^3 + 12ax^2 = 0 \implies 4x^2(x + 3a) = 0
with that done, what are the solutions to the equation
@urban copper you have 4 * x^2 * (x + 3a) = 0
for them to multiply to 0, that would mean at least one of the factors is 0
that means 4, x^2, or (x + 3a)
?
you now have the equation 4x^2(x + 3a) = 0
what are the solutions to this equation?
so the intersections are at x1 = 0 and x2 = -3a
the area lies between these intersections
so which intersection is the left end of the area?
thats correct
the right end of the area ends at 0
so horizontally, this area spans from -3a to 0
vertically, this area is bounded by y = a and y = 4x^3 + 12ax^2 + a
which one is at the top?
(one of them will always be on the top and the other one always be at the bottom, so you can test out values of a and x to see which one it is)
@urban copper are you stuck again
second one is on the top
we can graph them to be sure
but i only have access to pen and paper
I told you to test out values, did you do that?
i did
did it confirm that the second one is on the top?
what do you think value testing is for in calculus?
unsure tbh
youve seen that y = 4x^3 + 12ax^2 + a and y = a only intersect in two places
so that means, between x = -3a and x = 0, that one of them is always on top and the other is always on the bottom
compared to other methods, just placing in numbers for a and x will show which one is on top
this has proven that, when one value is tested, the entire region is confirmed
its also easier to just compare numbers than having to judge 4x^3 + 12ax^2 + a's behavior
are you with me so far?
no
do you want to elaborate?
sure
Im going to assume you didnt test the values properly
lets try some values like a = 1
that would mean y = 4x^3 + 12x^2 + 1 and y = 1
now we know the intersections are between x = -3 and x = 0
from using a = 1 with -3a and 0
I didnt use any theorems
dont delete your comments
thats bad practice to hide your mistakes
anyways, I can then test x = -1 to see which one is higher
y = 4(-1)^3 + 12(-1)^2 + 1 = -4 + 12 + 1 = 9
so we've seen that y = 4x^3 + 12x^2 + 1 is higher than y = 1 at a=1, x=-1
are you with me so far?
x=-1?
it doesnt matter which value of x I test
so long as that value is between x=-3 and x=0,
this value would be within the region where the area should be
and in that region, either 4x^3 + 12x^2 + 1 or 1 is higher
you didnt let me finish, either, on why the value ultimately doesnt matter
I chose x = -1 out of these values because it is the easiest to calculate
are we ready to continue?
okay
now lets use a property
not a theorem yet, just a property
i was just confused because you used a = 1 and x = -1 but okay
dont read into it too much
it was just too abrupt
everyone moves at a different speed, dont worry aobut it
sure
anyways
in general, we know that both y = 4x^3 + 12ax^2 + a and y = a are both continuous
continuity promises some nice things, one of them being IVT
continuity is still there if I set a = 1, so y = 4x^3 + 12x^2 + 1 and y = 1
now we've seen that they intersect only at x = -3 and x = 0
they can't intersect anywhere else
still with me?
(I havent gotten to anything important yet, just asking)
yes
now at x = -3 and x = 0, y = 4x^3 + 12x^2 + 1 and y = 1 are at the same height
(they intersect)
at x = -1, y = 4x^3 + 12x^2 + 1 is higher
lets assume that at x0, y = 4x^3 + 12x^2 + 1 is lower
with the additional restriction that -3 < x0 < 0
if this x0 is found, we would be able to prove that just testing one value isnt enough to rule the entire -3 < x < 0 region as "y = 4x^3 + 12x^2 + 1 is higher"
now for this to happen, 4x^3 + 12x^2 + 1 would need to lower and dip below y = 1
however, that would mean it would need to cross y = 1
which can only happen at x = -3 and x = 0
so the only way 4x^3 + 12x^2 + 1 can be below y = 1 at x = x0 is if x0 is outside of -3 < x < 0
since x0 can never be inside -3 < x < 0, that means y = 4x^3 + 12x^2 + 1 is always above y = 1 in the region -3 < x < 0
do you understand so far?
sure
not a yes
how does ivt come into play
is this a "yes" or a "suuuure"?
i agree
IVT never comes into play, because you didnt ask for a formal proof
we can use IVT along with a few modifications to prove this line here
since continuity already is pretty clear visually, you dont need a proof to see how the proof would work
how to prove
there you go
you need to keep that rigor throughout next time
now do you remember IVT?
yep
now a more general proof shouldnt need to depend on a = 1
I chose it as an example, so lets let a > 0 instead
this clashes with the a being used in IVT though
so instead you can read that as "continuous on [b, d]"
now with a being general, we found the intersections of y = 4x^3 + 12ax^2 + a and y = a at x = -3a and x = 0
now as before, I test a value out and do x = -a
since a > 0, then -a < 0,
since 1 < 3, then a < 3a, so -a > -3a
hence -3a < -a < 0
so x = -a is inside (-3a, 0)
do you understand so far?
sure
still not a yes
anyways lets see what happens if x = -a is tested
y = 4(-a)^3 + 12a(-a)^2 + a
y = -4a^3 + 12a^3 + a
y = 8a^3 + a
since a > 0, 8a^3 > 0
adding a to both sides, 8a^3 + a > a
y = 8a^3 + a is greater than y = a
so y = 4x^3 + 12ax^2 + a is higher than y = a at x = -a
do you get that?
you can also just say the word "yes"
ye
because the word "sure" is a universal sign of sarcasm so you cant use it in a legit way unless said in person
in short, I dont hear the tone you use when you use it
now let b be -a and -3 < d < 0 (and also f(x) = 4x^3 + 12ax^2 + a)
the theorem here assumes that b < d, so Ill need to repeat this again for when d < b
we know f(b) = f(-a) > a
if f(x) dips below y = a, then f(x) would < a
so lets assume f(d) < a
now let k = a
by IVT, there exists a constant in [b, d] = [-a, d] such that f(c) = k = a
however if f(c) = a, then y = f(x) and y = a intersect at x = c
but since -3 < -a = b < c < d < 0, c is not -3 nor 0
this would prove that c is somehow a third intersection which cannot happen, since we've already seen f(x) only intersects with a at x = -3a and x = 0
(also if f(d) = a, then the same "third intersection occurs but with x = d, so f(d) = a cannot happen)
so by contradiction, f(d) > a
so f(x) > a for all x in [-a, 0)
do you understand?
too much words and little latex gimme a minute to read
alr
latex would make this harder since you dont get to parse grammar
if you want, I can give you a latex version, but by the time I finish you'd probably get what I typed
I would appreciate it i am not in a hurry btw and i enjoy math
can you read $\forall$ and $\exists$?
mtt
universal and existencial quantifiers?
you have them swapped (yea thats correct)
and theyre read "for all" and "there exists"
anyways the latex here is going to use a lot of these since theyre short
@urban copper here you go
yes, I said earlier that the IVT is a bit one-sided
it assumes b < d
I then would have to post a similar proof for when d < b
the proof is very similar in that other than changing d < b, none of the machinery changes, so its not worth posting two very similar images if focusing on reading one is good enough
when you understand this proof, Ill post the second one
I also said earlier to change [a, b] to [b, d]
it looks like theres an unfinished line in the middle there
removed them
you can see how similar these are
brb
@urban copper back, you here?
@urban copper Has your question been resolved?
here
did you read the proofs?
only first one
you still need time reading them?
I think maybe math is not for me
formal proof latex is not for you
mostly because you werent taught formal proof latex
that is why you take english when you get it
it has grammar and words to connect nouns and phrases together
asking for a latex proof has no guarantees of being smoother than the words
fair
now how familiar are you with IVT
little to nothing
right, thats not good
to be able to understand how IVT is used, youll need to be familiar with what its saying
do you want to get into that or just take as a fact that f(x) > a for all x in (-3a, 0)?
lets assume it
oh
then lets move on
we've verified that the top of the region is always y = 4x^3 + 12ax^2 + a and the bottom is y = a
it starts on the left at x = -3a and ends on the right at x = 0
this is enough to write an integral of the region in particular
do you know how to write that integral?
sure
sounds good, try it out
,, \int_{-3a}^{0} 4x^3 + 12ax^2 dx - \int_{-3a}^{0} a \ dx
938c2cc0dcc05f2b68c4287040cfcf71
you mean $\int_{-3a}^0(4x^3+12ax^2+a)\dd{x}-\int_{-3a}^0a\dd{x}$
mtt
sure
usually this is memorized as $\int_{-3a}^0((4x^3+12ax^2+a)-(a))\dd{x}$ instead
mtt
so you directly consider it as the difference or gap between the top and bottom
now what
now you simplify and solve that integral
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x}
Hi
938c2cc0dcc05f2b68c4287040cfcf71
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0}
938c2cc0dcc05f2b68c4287040cfcf71
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0} \ &\implies \left[0 - \left(-3a\right)^{4}\right] + \left[ 0 - 4a\left(-3a\right)^3\right]
938c2cc0dcc05f2b68c4287040cfcf71
thats correct
now simplify what you have down there
youre kidding me
please do that by hand
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0} \ &\implies \left[0 - \left(-3a\right)^{4}\right] + \left[ 0 - 4a\left(-3a\right)^3\right] \ &\implies -\left(81a^4\right) -4a(-27a^3)
938c2cc0dcc05f2b68c4287040cfcf71
for a timesave, you can split 81 into 3 * 27
sure
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0} \ &\implies \left[0 - \left(-3a\right)^{4}\right] + \left[ 0 - 4a\left(-3a\right)^3\right] \ &\implies -\left(81a^4\right) -4a(-27a^3) \iff 81 = 3 \times 27 \ &\implies -3(27a^4) -4a(-27a^3)
938c2cc0dcc05f2b68c4287040cfcf71
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0} \ &\implies \left[0 - \left(-3a\right)^{4}\right] + \left[ 0 - 4a\left(-3a\right)^3\right] \ &\implies -\left(81a^4\right) -4a(-27a^3) \iff 81 = 3 \times 27 \ &\implies -3(27a^4) -4a(-27a^3) \ &\implies -3(27a^4) + 4(27a^4) \implies (27a^4)(-3+4)
938c2cc0dcc05f2b68c4287040cfcf71
@urban copper are you going to finish that
okay
,align &\implies \int_{-3a}^0\left(4x^3+12ax^2\right)\dd{x} \ &\implies \int_{-3a}^0\left(4x^3\right)\dd{x}+\int_{-3a}^0\left(12ax^2\right)\dd{x} \ &\implies \left[ \frac{4x^4}{4} \right]{-3a}^{0} + \left[ \frac{12ax^3}{3}\right]{-3a}^{0} \ &\implies \left[0 - \left(-3a\right)^{4}\right] + \left[ 0 - 4a\left(-3a\right)^3\right] \ &\implies -\left(81a^4\right) -4a(-27a^3) \iff 81 = 3 \times 27 \ &\implies -3(27a^4) -4a(-27a^3) \ &\implies -3(27a^4) + 4(27a^4) \ &\implies (27a^4)(-3+4) \ &\implies 27a^4
938c2cc0dcc05f2b68c4287040cfcf71
27a^4 is correct
Im pretty sure you mistyped it when you put that into the calculator earlier
now you know the area is 27a^4
according to the problem, what must this area also be equal to?
,tex \begin{enumerate}
\item Find all values of $a > 0$ such that the area enclosed by the line $y = a$ and the graph of $f(x) = 4x^3 + 12ax^2 + a$ is equal to $3a^2$.
\end{enumerate}
938c2cc0dcc05f2b68c4287040cfcf71
,align &\iff 27a^4 = 3a^2 \ &\iff a^2 = \frac{3}{27} \ &\iff |a| = \sqrt{\frac{3}{27}} \ &\iff a = \pm \sqrt{\frac{3}{27}}
yep, now solve for a
not with a calculator
thats not ideal
try square rooting instead
938c2cc0dcc05f2b68c4287040cfcf71
whats 3 / 27?
sure
,align &\iff 27a^4 = 3a^2 \ &\iff a^2 = \frac{3}{27} \ &\iff |a| = \sqrt{\frac{3}{27}} \ &\iff a = \pm \sqrt{\frac{3}{27}} \ &\iff a = \pm \frac{1}{3} \ &a > 0 \implies a = \frac{1}{3}
938c2cc0dcc05f2b68c4287040cfcf71
yep
by coincidence*, when a = 1/3, the area is also 1/3
so a, the area, 3a^2, and 27a^4 are all 1/3
how so
.close
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,tex \begin{enumerate}
\item Determine all values of $x \in \mathbb{R}$ for which the series $\sum_{n=1}^\infty \frac{2^n x^n}{n + 5^n}$ is convergent.
\end{enumerate}
938c2cc0dcc05f2b68c4287040cfcf71
there's the 'formal' way and then there's the easy way. the easy way is to just see that 2x < 5
formal I would prefer otherwise my homework solution might not be rigurous enough
<@&286206848099549185>
you can use the root or ratio tests
938c2cc0dcc05f2b68c4287040cfcf71
,tex [
L = \lim_{n \to \infty} \sqrt[n]{|a_n|}.
]
The Root Test states:
\begin{enumerate}
\item If $L < 1$, then the series $\sum_{n=1}^\infty a_n$ converges absolutely.
\item If $L > 1$, then the series $\sum_{n=1}^\infty a_n$ diverges.
\item If $L = 1$, the test is inconclusive, and the series may converge or diverge.
\end{enumerate}
938c2cc0dcc05f2b68c4287040cfcf71
,align a_n &= \frac{2^n \cdot x^n}{n + 5^n} \ \lim_{n \to \infty} \sqrt[n]{\left|a_n\right|} &= \lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|}
938c2cc0dcc05f2b68c4287040cfcf71
and we want it to converge
,, \lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1
938c2cc0dcc05f2b68c4287040cfcf71
@urban copper Has your question been resolved?
help me
wat is x lol
squeeze thm
$x \in \mathbb{R}$
938c2cc0dcc05f2b68c4287040cfcf71
use squeeze theorem to find limit
I think ratio test is easier to do
I want to practice ROOT TEST
Well your function is always positive so you can remove absolute value
yeah use squeeze theorem to find the limit of the sequence in the root test
nah im dumb as f sometimes
i wrote 2 + 2 as 5
in one exam
wdym
can u elaborate
n >= 1 so 2^n, n, 5^n is always positive
right
So you can reduce it to $\lim_{n \to \infty} \sqrt[n]{\frac{2^n |x^n|}{n + 5^n}}$
Closer
And I think you can split case to x >= 0 and x < 0 
Simply it first
,, \lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1 \ \lim_{n \to \infty} \sqrt[n]{\frac{2^n \cdot |x^n|}{n + 5^n}} < 1 \ \lim_{n \to \infty} \frac{\sqrt[n]{2^n \cdot | x^n |}}{\sqrt[n]{n + 5^n}} < 1 \ \lim_{n \to \infty} \left(\sqrt[n]{2^n} \cdot \sqrt[n]{|x|^n}\right) < \lim_{n \to \infty} \left(\sqrt[n]{n + 5^n}\right) \ |x| < \lim_{n \to \infty} \sqrt[n]{n + 5^n}
can u help or no
@white dagger
$\sqrt[n]{x^n} = x$
,, \lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1 \ \lim_{n \to \infty} \sqrt[n]{\frac{2^n \cdot |x^n|}{n + 5^n}} < 1 \ \lim_{n \to \infty} \frac{\sqrt[n]{2^n \cdot | x^n |}}{\sqrt[n]{n + 5^n}} < 1 \ \lim_{n \to \infty} \left(\sqrt[n]{2^n} \cdot \sqrt[n]{|x|^n}\right) < \lim_{n \to \infty} \left(\sqrt[n]{n + 5^n}\right) \ 2|x| < \lim_{n \to \infty} \sqrt[n]{n + 5^n} \ 2|x| < \lim_{n \to \infty} \sqrt[n]{5^n\left( 1 + \frac{n}{5^n}\right)}
938c2cc0dcc05f2b68c4287040cfcf71
,align &\lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1 \ &\lim_{n \to \infty} \sqrt[n]{\frac{2^n \cdot |x^n|}{n + 5^n}} < 1 \ &\lim_{n \to \infty} \frac{\sqrt[n]{2^n \cdot | x^n |}}{\sqrt[n]{n + 5^n}} < 1 \ &\lim_{n \to \infty} \left(\sqrt[n]{2^n} \cdot \sqrt[n]{|x|^n}\right) < \lim_{n \to \infty} \left(\sqrt[n]{n + 5^n}\right) \ &2|x| < \lim_{n \to \infty} \sqrt[n]{n + 5^n} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n\left( 1 + \frac{n}{5^n}\right)} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n} \cdot \lim_{n \to \infty} \sqrt[n]{\left(1 + \frac{n}{5^n}\right) }
$\lim{\sqrt[n]{1 + \frac{n}{5^n}}}$
Closer
And this will make you go around the circle, though
938c2cc0dcc05f2b68c4287040cfcf71
elaborate
I think you need to use the log trick here which is: $\lim_{n \to \infty}e^{\frac{1}{n} \ln{(n + 5^n)}}$
Closer
And use L'Hopital rule
let me handle this
,align &\lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1 \ &\lim_{n \to \infty} \sqrt[n]{\frac{2^n \cdot |x^n|}{n + 5^n}} < 1 \ &\lim_{n \to \infty} \frac{\sqrt[n]{2^n \cdot | x^n |}}{\sqrt[n]{n + 5^n}} < 1 \ &\lim_{n \to \infty} \left(\sqrt[n]{2^n} \cdot \sqrt[n]{|x|^n}\right) < \lim_{n \to \infty} \left(\sqrt[n]{n + 5^n}\right) \ &2|x| < \lim_{n \to \infty} \sqrt[n]{n + 5^n} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n\left( 1 + \frac{n}{5^n}\right)} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n} \cdot \lim_{n \to \infty} \sqrt[n]{\left(1 + \frac{n}{5^n}\right) } \ &|x| < \frac{5}{2} \cdot \lim_{n \to \infty} \sqrt[n]{1 + \frac{n}{5^n}}
938c2cc0dcc05f2b68c4287040cfcf71
Wait
I mean: $\lim_{n \to \infty}\sqrt[n]{n + 5^n} = \lim_{n \to \infty}e^{\frac{1}{n} \ln{(n + 5^n)}}$
Closer
squeeze theorem 1 < 1 + n/5^n < 1 + 5^n/5^n = 2 
Or you can use this
😂
Wait a minute what about n-th root
Inequality is preserved when u take n-th root and limit of a constant to nth root is 1
can u help or not
do u know squeeze theorem
y
u just can use it to justify why the limit is 1
u show the latex i can wait
,align &\lim_{n \to \infty} \sqrt[n]{\left|\frac{2^n \cdot x^n}{n + 5^n}\right|} < 1 \ &\lim_{n \to \infty} \sqrt[n]{\frac{2^n \cdot |x^n|}{n + 5^n}} < 1 \ &\lim_{n \to \infty} \frac{\sqrt[n]{2^n \cdot | x^n |}}{\sqrt[n]{n + 5^n}} < 1 \ &\lim_{n \to \infty} \left(\sqrt[n]{2^n} \cdot \sqrt[n]{|x|^n}\right) < \lim_{n \to \infty} \left(\sqrt[n]{n + 5^n}\right) \ &2|x| < \lim_{n \to \infty} \sqrt[n]{n + 5^n} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n\left( 1 + \frac{n}{5^n}\right)} \ &2|x| < \lim_{n \to \infty} \sqrt[n]{5^n} \cdot \lim_{n \to \infty} \sqrt[n]{\left(1 + \frac{n}{5^n}\right) } \ &|x| < \frac{5}{2} \cdot \lim_{n \to \infty} \sqrt[n]{1 + \frac{n}{5^n}}
938c2cc0dcc05f2b68c4287040cfcf71
Basically it says if u have a_n < b_n < c_n and lim a_n = lim c_n, then lim a_n = lim b_n = lim c_n
lim of 1^(1/n) is clearly 1
ok
lim of 2^(1/n) is also 1
,w limit of n to infinity of (1+ n/(5^n))^(1/n)
that's a little harder to prove if u don't already know it but uhhh the proof uses the binomial theorem
yeah I'm tryna remember the full proof lol
how to find x in R s.t. this shit converges
$|x| < \frac{5}{2}$ so $\frac{-5}{2} < x < \frac{5}{2}$
Closer
938c2cc0dcc05f2b68c4287040cfcf71
And you need to test whether the series convert at endpoint separately
Yeah, but maybe series converges at $x = 5/2$ or $x = -5/2$
Closer
So you need to test it
can u draw it and show me the latex
if a> 1 then a^(1/n) = 1 + b_n for b_n >= 0, so a >=1 + nb_n, so (a - 1)/n >= b_n >= 0, then squeeze theorem implies lim b_n =0 so lim a^(1/n) = 1
Plug x = 5/2 and x = -5/2 into original series
And check whether it converges at those points
can u draw it
Let me latex it
$\sum_{n=1}^{\infty} \frac{2^n (\frac{5}{2})^n}{n + 5^n}$ and $\sum_{n=1}^{\infty} \frac{2^n (\frac{-5}{2})^n}{n + 5^n}$
Closer
right
second one is alternating
due to (-1)^n
can someone help or not
lets apply root test for left series
to determine if it converges or diverges
after that we can play with the second one, which looks a little more intimidating
Bruh those two series are obviously
You don't need root test
Closer
938c2cc0dcc05f2b68c4287040cfcf71
so its 1/1
Obviously it is 1 or you can check with L'Hopital
Ok so it diverges at x = 5/2 so we exclude it
938c2cc0dcc05f2b68c4287040cfcf71
? 
The second one you can use alternating series test to see that it diverges. So the final result is $x \in (-5/2, 5/2)$
Closer
so its not inclusive
because both ends diverge
@white dagger
so are we done or ?
alternating series test
?
Yes, we are done
how to show it using leibnitz
Leibniz test?
alternating series test
but we need to show it is decreasing sequence
can we use lagrange?
It has two condition
Another condition is: $\lim_{n \to \infty}a_n = 0$
Closer
sure
,, \sum_{n=1}^{\infty} \frac{2^n \cdot \left(-1\right)^n \left(\frac{5}{2}\right)^n}{n + 5^n} \ a_n = \frac{2^n \cdot \left(-1\right)^n \left(\frac{5}{2}\right)^n}{n + 5^n}
$a_n = \frac{5^n}{n + 5^n}$
We don't care about (-1)^n
wdym
$\sum_{n=1}^{\infty} \frac{2^n (\frac{5}{2})^n}{n + 5^n}$ and $\sum_{n=1}^{\infty} \frac{2^n (\frac{-5}{2})^n}{n + 5^n}$
938c2cc0dcc05f2b68c4287040cfcf71
I was talking about the second series
I believe you are talking about the first one innit
@white dagger
You simply it
Then it will become: $\lim_{n \to \infty} (-1)^n \frac{5^n}{n + 5^n}$
Closer
Use second condition of alternating series test
oh it converges to zero
No it diverges
If you have alternating series: $\sum (-1)^n a_n$
Closer
Then for it to converges two conditions need to be satisfied:
1 is a_n need to form a decreasing sequence
2 is $\lim_{n \to \infty} a_n = 0$
Closer
Condition 2 is often easier to test first
its not
it’s not zero
$\lim_{n \to \infty} (-1)^n \frac{5^n}{n + 5^n}$
938c2cc0dcc05f2b68c4287040cfcf71
So it diverges
$a_n$ here is $\frac{5^n}{n + 5^n}$
Closer
Meaning it diverges
So -5/2 is excluded as well
why
is that a minimum condition
for any series
i DONT GET IT
@white dagger
That is alternating series test
but first one is not alternating
how can u
use alternating test for a NOT ALTERNATING SERIES
We are doing second one, though
Divergence test
???
If $\lim_{n \to \infty} a_n \neq 0$ then it diverges
Closer
sure
okay so we are done right
Yes. $x \in (\frac{-5}{2}, \frac{5}{2})$
Closer
sure
You can test with Wolfram alpha to be sure
My computation time excessed
,w SumConvergence[(2^n x^n)/(n + 5^n), n]
Ok so it is true
,w SumConvergence[(2^n x^n)/(n + 5^n), 1]
,w SumConvergence[(2^n x^n)/(n + 5^n),n]
@urban copper Has your question been resolved?
I can confirm $x\in\qty(-\frac52,\frac52)$ is the answer, so whats your remaining question
mtt
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This YouTube video says to factor a radical you should memorize the perfect squares which I have until 256
But when factoring how do I know the multiplication of those larger numbers
Do I start guessing with my calculator trying each perfect square
Like he shows 36 times 3 = 108
How would I know that
more practically, you dont memorize that that happens
what you do instead is repeatedly divide by potential factors until you break down the whole number
for example, I see 108 and its even
I try dividing by 2
108 = 54 * 2
108 = 27 * 2 * 2
27 is the third power of 3, I know that
108 = 3 * 3 * 3 * 2 * 2
108 = 3 * 3 * 3 * 2 * 2
then with this, you can see that pairs of the same number naturally form squares
3 * 3 * 2 * 2 = 3^2 * 2^2 = 6^2
so 108 = 3 * 6^2
so sqrt(108) = sqrt(3 * 6^2)
= sqrt(3) 6
Wow thank you so much
np
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Can someone help me in understanding the solution by providing a slightly more elaborate explanation?
938c2cc0dcc05f2b68c4287040cfcf71
but
if you take n^8 out of the sqrt is n^4
also
(n+1)^2 is to the power of two same with n^2
I got the first step thanks but I'm trying to wrap my mind around why the 2nd step was needed because this kinda seems like it's changing the equation too far than the original, for the limit
,, \frac{(n+1)^2}{n^2} = \left(\frac{n+1}{n}\right)^2
938c2cc0dcc05f2b68c4287040cfcf71
agree or disagree
AH THANKS
I couldn't figure out such a simple thing in the entire problem smh
Thanks a lot man
😭🙏
no worries, keep it hard
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hi
lim(x,y)->(0,0) (e^x-e^y)/x-y
i found this question somewhere
i asked my sir about it and he said its out of my syllabus yet i wanna know what the limit is
you can ping me
you assume t = x - y
(i haven't done anything like this but i think it is what question is really telling you to do)
e^y (e^(x-y) - 1) / (x-y)
so you have t and y
you get e^y = 1
and you know what (e^t-1)/t is
@atomic seal
...
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,tex \begin{enumerate}
\item Let ( f ) be a differentiable function that satisfies
[
x^2 f(x) = 3x + \int_{9}^{3x} t f\left(\frac{t}{3}\right) dt.
]
Find the Taylor polynomial of order 2 for ( f ) at ( x_0 = 3 ).
\item Find \( f \) differentiable such that
\[
f^3(x) f'(x) = 15 x^9 e^{2x^5}, \quad f(0) = 1.
\]
\item Find \( a > 0 \) such that the area of the region enclosed between the graphs of
\[
f(x) = a \sqrt{x} \quad \text{and} \quad g(x) = x^2 - 10x \quad \text{for} \quad 0 \leq x \leq 9
\]
is equal to 207.
\item Calculate the radius of convergence of the series
\[
\sum_{n=1}^{\infty} \left(\frac{n+1}{n+5}\right)^n \left(\frac{n+3}{2^n}\right) x^n.
\]
\end{enumerate}
938c2cc0dcc05f2b68c4287040cfcf71
2xf + fx^2 = 3 + 3x f - 9f(0)
can u draw it in latex
cmd?
do u have pen and paper
ok, write the latex then
fix RHS
チラチーノ
,, f(x) = f(3) + f'(3)(x-3) + 1/2! f''(3)(x-3)^2 + ...
チラチーノ
you need taylor polynomial of order 2
so you can ignore rest of it
you need f''(3) and f'(3)'s values
and f(3)
put x = 0
you get f(3) = 1/3
,, \item 2xf(x) + x^2f'(x) = 3 + 3x f(x) - 3
\item 2xf(x) + x^2f'(x) = 3x f(x)
チラチーノ
Compile Error! Click the
reaction for more information.
(You may edit your message to recompile.)
,, x^2f'(x) = x f(x)
チラチーノ
now you put x = 3 here
,,f'(3) = (3 \times 1/3)/3^2 = 1/9
how to write multiply symbol
$\times$
Pseudonium
ok thx
you get f'(3) = 1/9
you already got f(3) = 1/3
wait my calculations are bad
let me edit
please check your calculations
チラチーノ

now looks good
,, f'(x) = f(x)/x
チラチーノ
differentiate it again and put x = 3
you will get f''(3)
now just put there and you get the expression
(1st question solved)
,, f^{(2)}(3) = \frac{3f’(3) - f(3)}{9}
938c2cc0dcc05f2b68c4287040cfcf71
,, f’’(3) = \frac{\frac{1}{3}- f(3)}{9}
,, f^{(2)}(3) = \frac{\frac{1}{3}- f(3)}{9}
938c2cc0dcc05f2b68c4287040cfcf71
f’’(3) = 0
@urban copper Has your question been resolved?
@urban copper Has your question been resolved?
f(3) = 1. $3^2f(3) = 33 + 0$ (integral from 9 to 3*3 = 9 is 0), so f(3) = 1
nah I'm on mobile lol
992DuckEnthusiast (duck eater)
@urban copper
@forest vortex
@urban copper Has your question been resolved?
@urban copper what are you working on right now?
I'm confused, what part of the question are you stuck on
@urban copper Has your question been resolved?
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I need solve all questions
I can solve first order equations
But I haven’t tried the second order
I know few methods
Have you read your book or lecture notes?
I’m perfectly good till first order
you have to think of functions that have a derivative that is of the same type as the original functions
the exponential satisfies this requirement
If you attended the class of second order odes
or read the sectian that explains it
I didn’t attend unfortunately
I’m tensed
just a second
4 total problems and the time is running
I believe 1B 1C 1D follow the same pattern?
You can check for first order differential équations, you always need exp for things like y' = ay
but there's a catch
you only have 1 possibility for lambda in this equation
and there's a theorem that states
that a second order linear ode has 2 lambdas
After you found lambda, does the problem complete?
I feel overwhelmed
y0 and y'0
then all you can say is y(t)=Ae^(3t)+Bte^(3t)
how much time left do you have?
it's more than enough
Really?
yes
But I don’t wanna waste ur time
Are u okay with helping me throughout the entire assignment?
I can help you with some of the questions, the others are just a repetition
Where did A and B come from
Thanks bro
the ode is a linear one
so the solution is a linear combination of possible solutions
if you had 2 different lambdas you'd have y=Ae^(lambda1)+Be^(lambda2)
in the specific case of a repeated lambda, 3 and 3 in the first problem
you have to put a t, before the second solution
the independent variable
Kinda confusing tbh
Why do u have only one lambda here
A quadratic always has 2 solutions right ?
yes
but they are equal
the parabola touches the x axis in just 1 point
at the vertex
36-4*9=0
Makes sense the root is zero is here
yes
you have to substitute lambda 1 and 2 in the answer I sent you
Thanks for this formula
you're welcome
I’ll solve bc and send u the solutions
differential equations is my favorit topic
What does graph the solution in interval mean
It’s cool yeah
you have to plot the solution
and use y0 and y'0 to determine A and B
you have a system of linear equations
in this case you have 2 different roots
this is the correct formula for this case
the case where you have a 2nd order ode with 1 root on the polynomial equation is the special case
you've applied the wrong formula
use this one
just when you have just 1 lambda
substitute back A and B in the equation
and plot it
I'm helping you and another person, at discord too, in another language, at the same time.
No worries
This is way more exciting than being a teaching assistant, of students who don't even want to attend lectures.
Lmao I skipped so many classes
I hate summer classes
But you're asking for help now
My university went out of a strike recently. Our academic calendar is messed up.
That’s sad, can I copy the graph from a graphing calculator
I think so
Can u give me few mins
I need to copy 1a 1B to the answer sheet
4 more hours till deadline
6:33 pm in New York
@halcyon stratus finished copying?
Few more mins please I had some other work
ok
Sorry for making u wait
My fucking dasher canceled my order and I haven’t ate anything today
this is not healthy
go eat something
I'm at home
We in Brazil generally go to university at the city we live in, with our parents.
Then, solve it
Done copying
I’m moving on to c
I'm done with dinner
@halcyon stratus Has your question been resolved?
im having trouble with d
this is 16 not 6. and 4.5
ok
do u think we have time
its sqrt of 16
the complex roots are -1+2i
if u figured out 2,3,4 let me know
ok
,afk eating
calculating the 3a
C is solved
I belive that in 3a he is using Alpha and Beta instead of A and B as constants
