# Integral of Cos^2(x)

1. Jul 13, 2009

### Starwatcher16

Could someone walk me through this one? I know what the answer is, but don't really see how that answer is come by. In the book, it just states the answer, so I guess its something obvious, but the answer still eludes me :(

2. Relevant equations
$$\int{Cos^2{(ax)}}dx$$

2. Jul 13, 2009

### Cyosis

Make sure you know your trig identities.

$$\cos 2x=2 \cos^2x-1$$