#help-17
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If at t=0 I have 700 mCi of substance A, and the mCi of the substance A i 5% smaller than the mci of a substance B.
Then is it B=A+5%=735, or is it A=B-5*B/100 -> B=736.84....?
I think its the latter, but i wanted to make sure
please ping me when you answer
thank you
<@&286206848099549185>
guys ignore the mci and t=0 its just a precentage problem..
@mental cradle Has your question been resolved?
<@&286206848099549185>
b=a/0.95
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how did they separate ln3 from x/2lnx
$y = \lim a\cdot b^c = e^{\lim \log(a\cdot b^c )} = e^{\lim \log(a) + c\log(b)}$
riemann
identify a,b,c
riemann
oh is it because they dont share the same exponents
yea sure
thats the only way i see it making sense for me
so if im taking the ln of something and there's a coefficient in front of that something, i have to take the ln of the coeff as well?
,tex .log rules
riemann
product and then power
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I need help with this question. I don't know where to start.
so what i have thought about is looking for the area of the garden first
but i dont know if that is going to work
try setting one side as x and another side as y
then calculate the area in terms of those x and y
aswell as calculate the fencing in terms of x and y
@pulsar star
yes
so let x be the length and let y be the width
and make a function off those two sides to calculate area?
14W−W2
sorry i accidentally typed in my notes on the chat
dont bother opening a help channel if youre not going to be active
no im active im just stuck and dont know what to do
i told oyu what to do
write the area as a function of x and y
and the same for the perimeter
but for some reason you are ignoring my requests
not quite
there is a 1ft and 2ft border on both sides
2(2x + 4) + 2(2y + 2)
sorry im just not good at these types of word problems so it takes a while for me to think
4x and 4y?
2(x + 2) + 2(y + 1)
border thing again
yeah i just realized that
can you fix it?
(2x + 4) + (2y + 2) = 40
still same border issue
(2x + 4)(2y + 2) x 2 = 40
now youre just guessing
it helps to make a drawing
the perimeter is 2 * (x+2) + 2 * (y+4) = 40
and the area is x*y
knowing that do i just try to find x and y so i can put in the area function?
or since were maximizing
you first need to find y in terms of x from this
(or vice versa ofc)
y = A/x
$2x+4+2y+8=40$
Bonk
$y=?$
Bonk
oh so since maximizing we are expanding to make a quadratic function
this is just the perimeter first
ok i think i can do this part
because right now, we have two unknowns with x and y
but we can represent y in terms of x so make it just one unknown
are you able to do that?
y = -(x - 17)?
im not sure how oyu got a negative number
!show
Show your work, and if possible, explain where you are stuck.
oh shoot
actually, the rest does look correct
and if you change it to the right numbers
you get y=-(x-14)
now, fill that into area = x*y
so A = x*-(x-14)
do i expand the function into -x^2 +14
-x^2+14x, yes you could do that
how do you maximise?
complete the square find the biggest possible x value from the function
nothing with the derivative?
uh the problem is i have never been taught derivatives from grade 12 precalculus and college precalculus
youre grade 12?
im a freshman college student first year and i dont remember learning derivatives at all
yeah im going to look into it
,w graph -x^2+14x
to find the maximum of this
you take the derivative of -x^2+14x
which is -2x+14
and set it equal to 0
x = 7
7
and now what is the maximum area?
49 ft^2?
i guess so thank you very much or until the next question
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Hello, i have a question so say you are proving a result such as any prime number + x is a compositie number, could you use the logic that prime numbers are all odd except for two and just state that like 2n + 1 (prime) + x
but there's exactly one even prime number
yeha thats why i split it into cases no
my logic is like 2n + (1+x) where x+n is an even number
so then its ognna be even
with the gase of 2 it works as well
what exactly is x?
uh its for an assignment so i just wanted to ask a general question
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ok x = 7
you can also just use the general rule of adding parity
whats that
odd + odd = even, even + even = even, even + odd = odd
ok so thats what i was thinking
id split it up into cases tho right cause one for everything but two
and one for 2
if you haven't covered these yet, you can prove any of them using the same sort of logic as above
no i know
yes, you'd have to prove the case for 2 separately
alright thx
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😱
not all the adding at the end ofc just the start
Is that a yes 🤞
Bro i just started learning integrals, sorry
@vast shale mind helping this lad?
Please do not ping individual helpers unprompted.
Oh mb
first of all, where did 2pi come from. What are you even trying to solve?
It’s part of the shell method formula
like the OG OG question
Yea I sent the wrong pic mb
something feels wrong for your q2
Do you know what?
Bro do you know how to do this stuff😭
Ur trolling me
it’s part of the formula
Just like 2 pi is
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WHAT?
I need to you to come back here!
aight
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ahhh I'm an idiot
I'm pretty sure y = 1 should not be my solution
Ok, I'm 1/00 sure
Cause y(0)=1 is my condition I'm given
Ooohhh
I forgot to add c and divide that e term there
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✅
What da heck? What's this D_x?
What's D?
I understand that's the derivative wrt x, but of what?
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vieta's formulae
can't find a pattern here
i've worked out the sum of roots, product pairs of roots and product of roots
something to do with comparing the expanded form of (x-alpha)(x-beta)(x-gamma) with the given cubic equation
i feel that would yield information about alpha beta and gamma which you could use to construct a cubic equation for the given roots
Elaborate?
solving the cubic could also be a viable method
you can factor out x-1 since all the coefficients add up to 0
Yeah no I'm explicitly not trying to solve the cubic
It's either through vieta's or substitution
it might be difficult since you need to get the value of the roots to construct the second equation
I asked deepseek for this question
It also gave me a hint
The issue is that deepseek is down right now
So I can't ask it to solve this
I asked it to give me this question ages ago
if that's the case, here's a little more help
i just hate LaTeX so i always use desmos
don't judge
This is what I'm doing when I'm adding the roots
But to be fair I don't know if my new question is degree 3
Hmm
it should be
since there are three roots
Also true
So this is -b/a
for the product pairs and product, you have the value of abg and a+b+g
too lazy to type of the greek letters
I'm making a joke
i don't get it :<
abg also means something else
Search it up
Well anyways let's keep this maths related
main issue is finding the values of the square roots and square root pairs
not sure how you're going to solve it without actually finding out the values of the roots
I don't know if the hint is helpful
What does (root(alpha) + root(beta))^2 isn't helpful since it still has a radical
(root(alpha) + root(beta))^4 ?
Not sure if that has a radical
it will
What about ^3?
just think it through the binomial expansion
there must be a root alpha ^ 1 and root beta ^1 term in there for any expansion
Oh lmao
Yes I get that
How is the hint from deepseek helpful then?
probably isn't
me thinks
it is if you limit yourself to specifically not finding out the value of the roots
there just seems like no way to eliminate the radicals
and since we don't know their values (we choose to stay ignorant about it) we cannot substitute their values either
It does work out apparently chatgpt is answering it but mobile latex sucks so I can't tell
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when should i use shell method instead of disk/washer
When there is radial symmetry
!occupied
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what does that mean
Do problems and you will see what that means and when to use it
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my like is so cooked Im right now awake at a time when i should be sleeping snd im sleep at a time when the day i wake up is the same day
Such is life
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So I've gotten to this question and solve in by first replacing the value by
k/9 * k-1/9-1 (second time) = 7/12
by multiplying them together I got (k^2 - k)/81 = 7/12
Now doing crisscross I got 12k^2 - 12k = 637
After sending the 637 to the other side I now got: 12k^2 - 12k - 637 = 0
The next was supposedly me dividing every value by 12 but I got stuck at 637
you are having trouble dividing 637 by 12?
Nope
hang on
what is a and b
ok wait hold on
how did 637 even show up
i think you messed something up
wait also
the 81 is sus as well
$\frac k9 \cdot \frac{k-1}{9-1} = \frac{k^2-k}{72}$
ann.in.a.teacup
Tysm 🙏
This solved it
504 can be divided for 12
I just messed up between 8 and 9
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hi
hello, whats your question?
I'm supposed to find the largest range and the domain
and i'm pretty sure any odd multiples of pi/2 aren't even included in the domain of tanx
but idk.
So thats why im asking
And if they arent included, as im supposing, would i write it down like this?
also the range of tanx is all the reals right???
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can u like solve it on paper or a note to show me with details
well to be honest i have no idea why the square root is ther
but whenever you have the form
x^2 - a^2
then youd be able to factorize it by just
(x - a)(x + a)
ik how to solve others but am stuck on this one
some ppl said its multiplied by 1/2
$x-9=\left( \sqrt{x} \right)^{2}-3^{2}$
look
TargetVN
ty
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ik this is lame
the rule says ∫ f'(x)/f(x) dx=ln∣f(x)∣+c right ?
ok then if 1/xlnx dx
the derivative should be lnx+1 right ? using ((u'v+v'u)) because x is multiplied with lnx
but if you integrate it letting lnx be u
it gives ln∣lnx∣+c
which means in way or another that ∫ 1/xlnx dx = ln∣lnx∣+c so the derivative of xlnx is lnx ????????????????????????
"∫ 1/xlnx dx = ln∣lnx∣+c" this is correct
"the derivative of xlnx is lnx" this one is irrelevant
the derivative of xlnx is lnx+1
yes yes this is what is confusing me
$\frac{1}{xlnx}$?
David
shouldnt the lnx+1 be in the numinator ??
why is the numenator is 1 ?
its like im admitting that the derivative is 1 ?
$\int\frac{1}{x\ln x},dx=\int\frac{\ln x+1-\ln x}{x\ln x},dx$
I mean
you can do that ig
TargetVN
it is legal
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If a taxicab charges x cents for the first 1/9 mile and x/5 cents for each additional 1/9 mile or fraction thereof, what is the charge, in cents, for a ride of y miles, where y is a whole number?
ambiguous question right? Lol
y is a whole number which means y could be 0?
idk why use whole numbers if the definition is ambiguous
x + (9xy - x)/5 would seemingly be the answer
if we have naturals (excluding 0)
but if we allow 0 then meh we have a piecewise function as an answer ig
I think you colud assume y > 0 because no one wil pay a taxi to ride 0 distance
just take whichever interpretation is more… natural
first off i can't help but remark what a weird pricing scheme this driver's got of charging by the ninth of a mile
but also, presumably y ≥ 1 since y=0 would mean you are not riding at all?
i guess
yeah okay fair thanks everyone
i just thought uh like if it's a math question
wouldn't it just be better to clarify
what "set" you're working over
[natural numbers](#help-17 message) can include or exclude 0
in reality math is full of ambiguity, but most of the ambiguity doesn’t matter :P
like here
you could just define it piecewise if you wanted to include 0
yeah 😭
yeah naturals aren't that bad
whole numbers historically used to be integers too iirc lol
and now they're naturals (?) seemingly
so like it's the worse word choice imo
should've just went with natural number 😭
but yeah if we're going full context-based then
y>=1
anyway okay
thanks again everyone
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hello
how does 3800+3800*1.8% can be rewritten as 3800(1+0.018)?
shouldnt it be 3800(1*0.018?)
Result:
68.4
,calc 3800(1+0.018)
Result:
3868.4
,calc 3800+3800*0.018
Result:
3868.4
@tight delta Has your question been resolved?
still waiting...thx
that's not an explanation, that's just a screenshot
i'll wait thanks for others to hppefully chime in
maybe read the screenshot ?
@tight delta Has your question been resolved?
@tight delta Has your question been resolved?
Have you read riemann answer?
Here
If you're not still satisfied, you should at least explain why you think it's multiplication instead of addition
Alright 👍
@civic otter just to explain....the question was "growth was 1.8%" which basically means 1.018 or 1+0.018 when converted to a decimal
that's it basically
you didn't give the question at all
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$\frac{1}{k+1} \leq \int_{k}^{k+1} \frac{dt}{t} \leq \frac{1}{k}$
how can i square this inequation ?
<rajel />
i mean the only part that isnt clear to me is getting the square root of the integrale
if the expressions are non-negative and the inequality is true to begin with then you can square
how can i square the integral
compute it first
should i initiate it to the previous primitive and then get them back to the integral ?
what?
$$\left( \int_{k}^{k+1} \frac{dt}{t}\right)^2$$
<rajel />
how to do this
compute the integral first
wdym by compute it ?
calculate it
its clearly $\ln(k+1)-\ln(k)$
<rajel />
abs value actually
so k > 1
now i should do $(\ln(k+1)-\ln(k))^2$ ?
<rajel />
yes
$\ln(k+1)^2 -2\ln(k+1) \cdot \ln(k) + \ln(k)^2$
what now ?
@bitter pilot
-2ln(k+1)ln(k)
yeah
then you have what you wanted
<rajel />
no i want the integral symbol to stay
what?
i want to find it without computing it
like having an expression inside the integral
instead of finding the value directly
like im expecting it to be: $\int \frac{dt}{t^2}$ or something like this
no that's wrong
you could do the following
since we know this is the value, differentiate it
differentiate it ?
so ur saying to go the other way around
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angle subtended at the centre is double the angle subtended at the circumference by the same arc
ang MOJ (obtuse) = 2 * 80
ang LOJ (obtuse) = 2 * 126
then work out x
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Whats an example of a function thats injective but not bijective, or one thats surjective but not bijective
What have u tried
Im just trying to understand the definitions
injective means each distinct x has a distinct y (x being inputs and y being outputs)
and surjective means each y has an distinct x
y = x²
then y = 4 has x = 2 or x = -2 so surjective means that a y has at least one x
Ok so, this has two imputs for one output which means its not surjective
not injective was correct
its not both right
injectivity is basically uniquness
the example i made f(x) = x² where f : R -> [0, oo) is surjective
oo
because for every y value you can find at least one x value that maps to it
Ok injective means for every unique input u have an unique output
so this is not injective
surjective means every output has only. 1 input
ohhh there is no only
it means there just is at least one input
If you defined f : R -> R then it's neither surjective nor injective. For example, surjectivity fails because take for example y = -4 there doesnt exist one x-value that maps to it.
Injectivity also fails because both x = -2 and x = 2 map to 4
And if you define f : [0,oo) -> R then it's only injective but not surjective
each x value maps onto a unique y value
but not every y value is being mapped, e.g. the negative numbers
ok
Because u can get every pos number as an output with a distinct input
"for every unique input there is a unique output" is just a function. an injective function is the other way around, for every unique output there is a unique corresponding input
as there are no more negitive cases
Ok just to clarify
Injective : Distinct inputs ahve distinct outputs
Ie, two imputs do not map to the same output
yes
Surjective: For every output there is at least 1 input
yes
key being at least, which is why its diffterent from injective
and bijective means both are true
yes
ok bet
That makes snese
sense*
I was missing the at least portion
so in ur example
y=x^2 on R -> R i not injective because u can have two inputs for the same output
yes
Its not surjective becase the output can never be negative
but if we did y = x^2 on (0,inf) -> (0,inf) it would be both
basically yea
as not there is only one out for each input, and every output has an input
Is another way of saying serjective that the range = codomain
yes. or if you did R -> [0,inf) it would be surjective but not injective, or on [0,inf) -> R it would be injective but not surjective
would add on that because for every negative number y in the codomain, there doesnt exist at least one corresponding element x of the domain
ok that adds up
Because from R -> [0,inf), multiple inputs give u the same output
and on [0,inf) -> R there is no way to get the negative numbers
ok bet, thanks for helping!
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hi guys, ive been staring at this for a hot minute and i still do not have a singular clue of what exactly is going on
bit of help would be appreciated
68=7*10-2 yes?
yes
yes
third from the bottom
after the arrow?
yes
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hello im studying for a test and im wondering what would i do to solve these questions, i kinda know how to do inequalities but really only one step ones. im tryna practice two step then get to multip step. here's my question:
Try to solve the equation first.
As in, make the > sign an = sign.
Then, you (at least for this level of inequalities) can test to see if it is true for x>root or x<root.
,w solve for x, -4x + 35 > 2
✅
yup
now im tryna do 8x+2>34
i moved the constant to the right and step 2, i have 8x > 34 - 2 right now
34/8 lets see if im right
subtracted 34
no wait
34 to 32
32/8 is what i meant to sayt
,w solve for x, 8x + 2 > 34
!done?
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I can already see them multiply by a negative number (since they are solving it as if it is an equation) and failing the test miserably.
Eh theyre suppsode to test
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is this false?
i dont think its right bc we go to 2n
which is ofc greater than n
so im pretty sure some remainders will repeat
or actually it might be true from testing values
it gives all the odd remainders
no
try for small values of n
well yeah, all odd except 0
yeah i have
and it does seem to work
but idk if im convinced
it seems to work though
is it true?
i think its like filling up the holes
yeah
like the first pass when youre still below n
first evens, then odds
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Certainly numbers <=2i are all distinct residues, the even residues specifically, with i=floor(n/2), and in fact 2i=-1 mod n. Then, 2(i+1)=2i+2=1 mod n, and the 2(i+a) for 1 < a < n-i are between 0 and n, and are all distinct. They're also all distinct from the numbers <2i since those are even and these are odd.
ty!
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is there any tangible difference to using square bracket for matrix over parentheses?
i have seen some people saying that square bracket is for physicians and engineer, while the latter is for mathematicians
purely a style choice
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@worthy ferry Has your question been resolved?
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Is there away to limit this function and transform it to another place?
for limit, you can put conditions
e.g. { 0 < x < 1, 0 < y < 1 }
to shift it around, apply horizontal and vertical shift
that is, adding constants to each x and y term
Ye but it doesnt work with this function
probably because it's too complex
show what you tried
Is there a way to move it around?
Wait actually it does now but i think it was lagging yesterday

f(x - a, y - b) shifts the function a unit rightwards, b unit upwards
you can see desmos is even complaining the function is too complex to display all the details
it will take some time, but it should work
Where do we put this in?
let's say whatever equation you have is x^2 + y^2 = 9
to shift the function a unit towards the right, you minus each x term by a
(x - a)^2 + y^2 = 9
to shift the function b unit upwards, minus each y term by b
(x - a)^2 + (y - b)^2 = 9
do this with all x and all y
!done
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!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
@hushed loom Has your question been resolved?
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@hushed loom Has your question been resolved?
Have you split it into multiple integrals to get rid of the absolute value?
(If not, do you know how to do that in the first place?)
i do
Then do it here
itll get rid of the absolute value and just turn it into a pretty easy integral
yeah did
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on part c
it doesnt make sense
i keep getting
153.4
but
the markscheme answer is
110
ive done everything
show work
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So my first question already, the limit only is for 1/x?
aaah alright
because the top bound of the integral is x
and the integral is with respect of t
that's how you kno
w
@twilit needle Has your question been resolved?
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The integral is scary?
yes definitely for me at least
and the power 1/t
we are only allowed to use sin cos tan and their inverses :/
and we havent done a lot of integrals so far yet
i think i have done like 30 or 40 in my life
Have you done Taylor series?
ye we had that last week
So this is a Taylor expansion
For a small t approximate tan2t
Which will just be 2t
Does that look familiar?
(1+1/x)^x
The limit of this as x->inf is e
So I won’t bother to prove that
You can take that as a lemma
So when we inverse the variable inside the parentheses and the exponent we get e^-x
So what you end up with is e^-2 as t->0
So what you end up with is
$ \lim_{x \to 0} \frac{1}{x} \int_0^x e^{-2} dx$
Which is just e^-2
Easy
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How to get the limit as x stands to infinity: ln(e^2x -e^x +1)-1-2x+1
$\lim_{x \to \infty } \ln(e^{2x} -e^x +1)-1-2x+1$
This?
X is above with 2
TargetVN
Hint: $\ln(e^{2x} -e^x +1)=\ln\left( e^{2x}\cdot (1-\frac{1}{e^{x}}+\frac{1}{e^{2x}}) \right)$
TargetVN
Can you continue from here?
also -1+1 = 0, so the -1 and +1 cancel each other
$\ln(e^{2x} -e^x +1)=\ln\left( e^{2x}\cdot (1-\frac{1}{e^{x}}+\frac{1}{e^{2x}}) \right)=2x+\ln\left( 1-\frac{1}{e^{x}}+\frac{1}{e^{2x}}\right)$
TargetVN
split them out
np
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I think that I made a mistake
Don’t understand first line logic
isnt that IBP?
sin(t-s) = -cos(t-s)?
Oh wait double negative
yes
what's the correct answer
uhm 1/2(e^t-cos(t)-sin(t))
Alr, I'm getting just the negative of that
The calculus part is right
You just plugged in the bounds wrong
Hm?
You plugged in 0 and t wrong for s
i did?
yeah
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in part (a) why does the bounds has to be like that for the area to come out as positive...? mine first come out as negative and i dont get why? and will i lose points if i wrote the bounds like the negtaive one and stick the absolute sign at the end to make it as an area?
$\int^b_a f(x) \dd{x}=-\int^{a}_{b} f(x) \dd{x}$
Civil Service Pigeon
Putting the bounds in the “reverse” order means that you’re considering the area in a clockwise direction rather than a counterclockwise one
Which has an analogous problem to when you’re finding areas in the Cartesian plane and work from right to left instead of left to right
i originally tried to trace the area by going from a smaller theta to a larger theta but since that would make the upper limit < lower limit that means I will need to stick the negative sign? so i should put the larger theta on top? trace from larger to smaller? to make it positive?
i originally tried to trace the area by going from a smaller theta to a larger theta but since that would make the upper limit < lower limit
uh
what? whats wrong?
How are you getting upper < lower?
Lower limit is where you start
Upper is where you end
You’re starting from a smaller angle, so that should be your lower limit
And the larger angle that you end at should be your lower limit
i was originally doing pi/6 to 5pi/6 and 5pi/6 to 13pi/6
thats how i thought its gonna be like smaller to larger is easy to think
(a to b means a lower bound of a and an upper bound of b)
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hello,, if we suppose that (an) is a strictly positive number sequence. Let Sn be the sum of ak from 0 to n and we suppose that anSn -> 1. Show that the series of an diverges and that an -->0. I don't understand how to to do that pleas help me
We should probably start by writing all that we know down
dyxn
thx for lateX didnt know how to write it
Let me try some stuff out
the only thing I got so far is Sn*an>an^2>0
I guess a good way to start would be to assume Sa_n converges
Sn or an?
$\sum_{n=1}^{\infty}a_n$
NotAPortalGuy
ok thx
So Sn
Yeah I was able to build a proof pretty easily using contradiction for this
But I can't find a good proof for a_n --> 0
thats easy too
If this sum diverges
It must diverge to infinity
But
I think
Hm
Oh yes
It follows
dyxn
True
So lim (an)(sn)=(liman)(lim sn)=0(lim sn)=0
thx I think its good
But it still doesn't prove that a_n --> 0
Yes
yes but we made one step
But when we know Sn diverges
And to infinity
We can prove it directly from the definition of the limit
If S_n diverges to infinity you can't say anything about a_n though
oh yeah
ok I have it I guess
Yeah you can probably just do it with limit rules
if Sn diverges, let A be a stricly positive real number, then it exists N, like for every n>N; an|Sn|>Aan then an(sum of ak)>Aan, which is possible only if an converges to 0
is it good or not?
Fr
why?
Isn't that basically what I did but you've switched some words around
yes a bit but now we know that Sn diverges
so the only assumption is that an converges towards l different from 0. I can do it more cleanly by separating in the case an diverges and the case an converges
and if its different than 0, then it's contradictory
an(sum of ak)>Aan, which is possible only if an converges to 0
I dont get this sentence at all
Or rather part
well an(sum of ak) is an Sn so it converges to 1, but the inequality is true for every A
so the only case in which its possible is that A*an <1 for every A, so an must go to 0
idk if its clear, perhaps my understanding is wrong but to me it makes sense
ok thx!
@vast shale Has your question been resolved?
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Guys help me solve 3
Anyone ?
if you consider zooming in i might help you

