# Fourier series-desperately needing help

1. Sep 13, 2007

### kring_c14

fourier series--desperately needing help

1. The problem statement, all variables and given/known data

http://www.exampleproblems.com/wiki/index.php/FS1
2. Relevant equations

in the a$$_{n}$$ on the fifth equal sign, why was (-1 ^n)? how did he arrive at that equation?

have mercy..
its our quiz today
and i really have a hard time on fourier

3. The attempt at a solution

2. Sep 13, 2007

### Slaviks

cos (\pi n) for n = even integer is equal to 1,
while for n = odd inetger, it is equal to -1.
The function (-1)^n has the same property.
Since n is an integer and must be either even or odd,
we can replace cos (\pi n) with (-1)^n .