Primitive of Fourth Power of Cosine Function

Theorem

 * $\ds \int \cos^4 x \rd x = \frac {3 x} 8 + \frac {\sin 2 x} 4 + \frac {\sin 4 x} {32} + C$

Proof
From Primitive of $\cos^4 a x$:

The result follows by setting $a = 1$.