How to derive sine series without Taylor's formula?

In summary, the conversation revolves around deriving the sine power series without using Taylor's formula. The problem is from an undergraduate level and involves researching about Madhava and his contributions to the concept of a limit. One suggestion is to start with the inequality \sin x \leq x and use integration to prove the series, with the use of induction and the squeeze theorem.
  • #1
shanu_bhaiya
64
0
1. The problem statement
Derive the sine power series without using Taylor's formula.
 
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  • #2
Why?
 
  • #3
HallsofIvy said:
Why?
Well, this is what the question reads. Without using the calculus, is it possible to derive sine power series using geometry, etc.
 
  • #4
I think you need more information. Like what is given because I use the power series as a definition of what sine is. Is this a differential equation problem?
 
  • #5
My question was really, "why should I do it". As I am sure you are aware, you are expected to try it yourself first and show us what you have done.
 
  • #6
mistermath said:
I think you need more information. Like what is given because I use the power series as a definition of what sine is. Is this a differential equation problem?
This is the only standalone-statement of the problem. Nothing else is given. No it's not a dfn eqn problem. The question's standard is upto 1st year undergraduate (including Real Analysis) only.
 
  • #7
HallsofIvy said:
My question was really, "why should I do it". As I am sure you are aware, you are expected to try it yourself first and show us what you have done.
Well problem is that I've no idea from where to start, my guesses are geometry, complex number or the geometric definition of sine. Actually it's a part of my report, my main subject was Calculus of Kerela(India), the ancient Indian people knew the series without Taylor's formula, so that's what I'm intended to write in my report but it's nowhere on the internet, any suggestions are welcome.
 
  • #8
shanu_bhaiya said:
Well problem is that I've no idea from where to start, my guesses are geometry, complex number or the geometric definition of sine. Actually it's a part of my report, my main subject was Calculus of Kerela(India), the ancient Indian people knew the series without Taylor's formula, so that's what I'm intended to write in my report but it's nowhere on the internet, any suggestions are welcome.

You got me curious. You just aren't searching the internet widely enough. Add Madhava to your queries. There's plenty of stuff. It looks to me like he and his school invented the concept of a limit, yes, probably couched in a geometric language. But I don't think they did it in any fundamentally different way than it was done two centuries later in the West. As a side note, Archimedes was also doing "method of exhaustion" proofs (i.e. limits) circa 200BC. Interesting subject. Keep researching.
 
  • #9
Well I came up with this, but it uses some integral calculus. Should still apply though. It comes up with cosines series as well.

----

Start with the inequality [itex]\sin x \leq x[/itex] for all [itex]x\geq 0[/itex].
Integrate both sides of the inequality from 0 to some positive value t. Then you should get an inequality for cos t. In turn integrate that between 0 and x, you get one for sine again. You should get what to do now. Formalize the proof with induction and the squeeze theorem to really impress your teacher.
 
Last edited:

1. How do you derive the sine series without using Taylor's formula?

The sine series can be derived by using the Euler formula, which states that e^(ix) = cos(x) + isin(x). By substituting x with -ix, we get e^(-ix) = cos(-x) + isin(-x). Using the fact that cos(-x) = cos(x) and sin(-x) = -sin(x), we can rearrange the formula to get sin(x) = (e^(ix) - e^(-ix)) / 2i. This formula can then be expanded using the binomial theorem to get the sine series.

2. What is the motivation for deriving the sine series without Taylor's formula?

The Taylor series uses derivatives to approximate a function. However, this method requires the function to be infinitely differentiable, which is not always the case. By deriving the sine series without using Taylor's formula, we can expand the range of functions that can be approximated.

3. Is the sine series accurate without using Taylor's formula?

Yes, the sine series derived using the Euler formula is just as accurate as the one derived using Taylor's formula. Both methods rely on the same fundamental principles and have been proven to converge to the same value for any input value of x.

4. Are there any limitations to deriving the sine series without Taylor's formula?

One limitation is that the Euler formula only works for complex numbers, so it cannot be used to derive the sine series for real numbers. Additionally, the binomial theorem can only be used to expand the formula for certain values of x, so the series may not converge for all values of x.

5. Are there any real-world applications for deriving the sine series without Taylor's formula?

Yes, the sine series is commonly used in signal processing, physics, and engineering to approximate periodic functions. By deriving the series without Taylor's formula, we can expand its usefulness to a wider range of functions and improve accuracy in certain applications.

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