How do I solve the integral of cos^2(x) using trigonometric identities?

[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 :(

Homework Equations

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

 

[Cyosis]

Make sure you know your trig identities.

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