Primitive of Reciprocal of Square of 1 plus Cosine of a x

Theorem

 * $\ds \int \frac {\d x} {\paren {1 + \cos a x}^2} = \frac 1 {2 a} \tan \frac {a x} 2 + \frac 1 {6 a} \tan^3 \frac {a x} 2 + C$

Also see

 * Primitive of $\dfrac 1 {\paren {1 + \sin a x}^2}$