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

Theorem

 * $\displaystyle \int x^3 \left({\sqrt {x^2 + a^2} }\right)^3 \ \mathrm d x = \frac {\left({\sqrt {x^2 + a^2} }\right)^7} 7 - \frac {a^2 \left({\sqrt {x^2 + a^2} }\right)^5} 5 + C$

Proof
Let: