Primitive of Reciprocal of x fourth minus a fourth

Theorem

 * $\displaystyle \int \frac {\mathrm d x} {x^4 - a^4} = \frac 1 {4 a^3} \ln \left\vert{\frac {x - a} {x + a} }\right\vert - \frac 1 {2 a^3} \arctan \frac x a$