# Homework Help: Need help with a integral.

1. Apr 9, 2009

### inko1nsiderat

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

The problem is in the solution to a PDE, the coefficient for the Fourier series is of some form like an=$$\frac{1}{a}$$$$\int cos(nx)cos(x) dx$$ over the interval -a to a.

2. Relevant equations

Orthogonality relations:

$$\int cos(nx) cos(mx) dx =[ 0, n \neq m, pi n=m \neq 0$$

over the interval -pi to pi.
3. The attempt at a solution

Using the orthogonality relation I figured the answer would be 1/a*a for m=1, and 0 for $$n \neq m$$. However for a particular problem the book says there is a solution for $$n \neq m$$. Is there a better way to go about this particular integral?

2. Apr 9, 2009

### Billy Bob

If a is not pi, then you can't use the orthogonality relation. This assumes your integral in step 1 is set up correctly. To evaluate it, just find the antiderivative and use FTC. Any decent table of integrals will have the antiderivative. There are two cases, n=1 and otherwise.

3. Apr 9, 2009

### inko1nsiderat

Ah alright thanks very much.