Primitive of Reciprocal of x by Root of x squared plus a squared cubed

Theorem

 * $\displaystyle \int \frac {\mathrm d x} {x \left({\sqrt {x^2 + a^2} }\right)^3} = \frac 1 {a^2 \sqrt {x^2 + a^2} } - \frac 1 {a^3} \ln \left({\frac {a + \sqrt {x^2 + a^2} } x}\right) + C$

Proof
Let: