Primitive of Reciprocal of 1 plus Cosine of a x

Theorem

 * $\displaystyle \int \frac {\mathrm d x} {1 + \cos a x} = \frac 1 a \tan \frac {a x} 2 + C$

Also see

 * Primitive of $\dfrac 1 {1 + \sin a x}$