# Homework Help: Fouiers series Calculate sum

1. Nov 3, 2011

### dikmikkel

1. The problem statement, all variables and given/known data
Consider the 2$\pi$-periodic function f(t) = t t in [-Pi;Pi]
a) show that the real fouier series for f(t) is:
$f(t) ~ \sum\limits_{n=1}^{\infty}\frac{2}{n}(-1)^{n+1}\sin nt$
b)
Use the answer to evaluate the following : $\sum\limits_{n=1}^{\infty}\dfrac{(-1)^{n+1}}{2n-1}$
Hint: Use Fouier's law with t = $\pi$/2

2. Relevant equations
Fouiers Law? I'm danish, and therefore i'm not really sure what it's called.

3. The attempt at a solution
Part a i have done by finding the coefficients.
Part b) I can't see where the problem in part b and the answer to a relates. I've tried with Maple 15 to calculate the value and i'm getting Pi/4, but i keep getting something different for the series from a)

Last edited: Nov 3, 2011
2. Nov 3, 2011

### ahm_11

The answer should be pi/4 ...

from fourier series we got ...

t = $\sum\frac{2}{n}(-1)n+1sinnt$

for t = $\frac{\pi}{2}$ ,

$\frac{\pi}{2}$ = 2 - 2/3 + 2/5 - 2/9 + ......

which followed ,

$\frac{\pi}{2}$ = 2 * $\sum\frac{(-1)n+1}{2n - 1}$

hence, $\sum\frac{(-1)n+1}{2n - 1}$ = $\frac{\pi}{4}$

3. Nov 3, 2011

### dikmikkel

Hmm, isn't that a backwards proof? Can't i use something like the even terms of sin(n*Pi/2) = 0 and the odd ones alternates between -1 and 1

And how is t = ... isn't it f(t)?