Primitive of Cosine of a x by Cosine of p x

Theorem

 * $\displaystyle \int \cos a x \cos p x \ \mathrm d x = \frac {\sin \left({a - p}\right) x} {2 \left({a - p}\right)} + \frac {\sin \left({a + p}\right) x} {2 \left({a + p}\right)} + C$

Also see

 * Primitive of $\sin p x \sin q x$