Primitive of Reciprocal of x by x squared plus a squared

Theorem

 * $\displaystyle \int \frac {\mathrm d x} {x \left({x^2 + a^2}\right)} = \frac 1 {2 a^2} \ln \left({\frac {x^2} {x^2 + a^2} }\right) + C$