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Help - Derivation of formula for Pi (1 Viewer)

Sy123

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But you didn't sum up the probabilities. What you just did was find the probability of getting k heads out of 2n tosses, then made the number of tosses approach infinity.

Of course, if I take a million tosses, then the probability of getting say 20 heads/tails would approach zero.
I see, so it sort of balances out, the more tosses we make, the lower the probability, yet the more stuff we can add on so in the end it becomes 1 right?

Though the sum of the probabilities is independent of n anyway...(obviously)
 

Carrotsticks

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I see, so it sort of balances out, the more tosses we make, the lower the probability, yet the more stuff we can add on so in the end it becomes 1 right?

Though the sum of the probabilities is independent of n anyway...(obviously)
That is exactly correct =)
 

Sy123

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========================

Alright new problem:

I want to find



Where I can pick an appropriate value of theta such as pi/2 or something.

Now, here

So the problem simplifies to finding:



Using whatever elementary methods that I can muster, I decide to go through the geometric series approach -> using complex numbers and then somehow getting a result:
But first I needed to find HOW I could get such a sum, so I decided to use a calculus approach, if I differentiate the summand above in terms of theta



So that is basically my new goal, that is easy enough to acquire using methods of calculus on our geometric series:



Now we come across the first problem, I need to substitute z = cis theta, but here comes 2 rigour issues

1. Am I allowed to use u-substitution of complex numbers?

2. Am I allowed to equate real and imaginary parts WITHIN the actual integral?


I realise I probably can't justify 1 within HSC level though.
I considered definite integrals, however I need to retain that z on the LHS, so I can then divide by z.

Thank You. Other approaches to the problem are appreciated.
 
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Sy123

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I was able to get this now, after some integration, taylor forms of ln and some big stuff, I arrive at this identity, which technically we could find via observation but I got it anyway.



Now as shown on the first page, I can show that the first term becomes:
However I have been trying to do something similar for the second term and it doesn't work.

If I did something like I did for the first identity I proved in this post, and that is the integration FIRST then substitution, then using taylor form for ln and stuff

I get a loop where LHS = RHS and I just came around on myself :/

So the second term in that would finish this problem off. Help will be appreciated.

EDIT: I think I got the result, I just need to verify it

EDIT2: It was wrong and I did it again using another route without that 'identity' above which turns out to be not always true. I have the correct answer now:

 
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Sy123

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Is there anyway to find:

In terms of alpha preferably, and using HSC methods if possible (the most advanced I will accept is taylor series but even then I can't justify integration of infinite terms)

Thanks
 

Shadowdude

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iirc, if you express that as taylor series - you can only integrate it term by term if the series converges


And if the series converges you'll most likely get something icky to integrate.
 

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This question is beyond me. But I wonder if HSC Extension 2 2008 Q8a is any help?
 

Sy123

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This question is beyond me. But I wonder if HSC Extension 2 2008 Q8a is any help?
Q8a is just a proof of the result that I initially used, the background on this problem is:

I can prove that odd cosine thing result easily with geometric series (no need for induction as the HSC specifies)

I simply integrate twice after that:



Then integrate again this time picking appropriate bounds to directly arrive at:



I can arrive at the result that:



And we can then arrive at a solution to the Basel Problem:





So if I can find the RHS, I can find the LHS, which is the whole point of finding the integral
 
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Carrotsticks

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iirc, if you express that as taylor series - you can only integrate it term by term if the series converges


And if the series converges you'll most likely get something icky to integrate.
No. One can only safely integrate term-by-term infinitely many times if the term converges uniformly.

My fault, misread your comment regarding Taylor Series. In that case, you can only integrate it term by term if the interval of integration exists within the radius of convergence of the Taylor Series (though you might be lucky enough to have a Taylor series that has an infinitely large radius of convergence. For example, the inverse tan function has the radius |x|<1 whereas the sine function has the radius |x|<infinity.)

Is there anyway to find:

In terms of alpha preferably, and using HSC methods if possible (the most advanced I will accept is taylor series but even then I can't justify integration of infinite terms)

Thanks
It won't be a nice result if it is in terms of alpha only. If alpha were say pi/2 then yes a decently nice result can be deduced, but not with just alpha because we know that sin(pi/2)=1 whereas sin(1) doesn't exactly have a nice simple form.
 
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Sy123

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No. One can only safely integrate term-by-term infinitely many times if the term converges uniformly.

My fault, misread your comment regarding Taylor Series. In that case, you can only integrate it term by term if the interval of integration exists within the radius of convergence of the Taylor Series (though you might be lucky enough to have a Taylor series that has an infinitely large radius of convergence. For example, the inverse tan function has the radius |x|<1 whereas the sine function has the radius |x|<infinity.)



It won't be a nice result if it is in terms of alpha only. If alpha were say pi/2 then yes a decently nice result can be deduced, but not with just alpha because we know that sin(pi/2)=1 whereas sin(1) doesn't exactly have a nice simple form.
There is a typo in that question, I need sin(2ntheta)/2sin(theta)
Oh and I need one of the limits to be variable in order to then integrate once more :/ (in order to attempt to solve the Basel problem)
 

Sy123

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I might also add that doing this problem is equivalent to proving:


If I can prove this, then I end up with the value

Which is indeed the correct value of

But I haven't proved this, and I need help proving it.
 

Sy123

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I decided to focus on instead:



Wolfram Alpha outputs:



What does this even mean? csc(x) -1 to 1.....
 

Trebla

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since sin 2nx oscillates between -1 and 1 as you let n get large i.e. it doesn't converge
 

Sy123

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isn't that just the domain?
So its never a definite value? Just a range of values between csc(x)-1 and 1? (I guess this was predictable considering the functions are periodic)

But oh well :(
Any ideas on how to compute the integral?
 

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So its never a definite value? Just a range of values between csc(x)-1 and 1? (I guess this was predictable considering the functions are periodic)

But oh well :(
Any ideas on how to compute the integral?
I'll take a look at it tomorrow when I have time.
 

Sy123

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I'll take a look at it tomorrow when I have time.
That would be greatly appreciated, thank you.

==============

However I decided to tackle the problem from a different point, and I decided to change the limit of integration of alpha to 0, to alpha to pi/2. It has given some interesting results:



Now we will integrate the LHS, we end up with:



Then integrate once more with the limits that I already had then take n off to infinity, I end up with something very perculiar:



Now, if you can notice the second term on the left hand side is pi/4 (this can proved in various ways, as shown on the first page and otherwise). And you can notice a multiplier of pi to pi/4 making pi^2/4...

This means that IF we can prove that the RHS = 0. Everything falls into place very nicely here.

That:



===========

Now when trying to prove the RHS is zero, if I integrate by parts once, I get two terms the first term has a multiplier of 1/2n on the front
and the integral that we get also has a co-efficient of 1/2n at the front. Am I allowed to then assume that since n goes to infinity the terms converge to zero despite the fact that we have an uncounted definite integral?

If you integrate by parts you get:



Can I then say this all converges to zero due to the 1/2n as n approaches infinity?
 

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Yep, you can say that it indeed converges to 0 because as long as alpha isn't pi, the identity holds and we have a fixed value in the left hand term. If we have 1/2n times some fixed value, then ofc as n gets large, the whole term -> 0.

Can you now perhaps show a full proof so I can check to see if all steps are legal? Doesn't need to be typed, handwritten will do.
 

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Assuming what you did was correct, I think it does work out. Note that



so by squeeze law it should converge to zero (which still holds with whatever multiplicative constant is attached). Likewise for the integral:



again by squeeze law the integral converges to zero.

NOTE: One thing I need to add is that multiplication of the cosine on all sides of the inequality doesn't guarantee the preservation of inequality sign. However, whether or not the inequality signs switch does not change the convergence.
 
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Sy123

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Yep, you can say that it indeed converges to 0 because as long as alpha isn't pi, the identity holds and we have a fixed value in the left hand term. If we have 1/2n times some fixed value, then ofc as n gets large, the whole term -> 0.

Can you now perhaps show a full proof so I can check to see if all steps are legal? Doesn't need to be typed, handwritten will do.
Here we go:

Part 1: Developing the identity

Part 2: Integrating finding a form for Basel problem

Part 3: Evaluating the integral

Assuming what you did was correct, I think it does work out. Note that



so by squeeze law it should converge to zero (which still holds with whatever multiplicative constant is attached). Likewise for the integral:



again by squeeze law the integral converges to zero.
That is the better way to do it lol, is my one sufficient though?
 

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