Primitive of Reciprocal of Root of x squared plus a squared

Theorem

 * $\displaystyle \int \frac {\mathrm d x} {\sqrt {x^2 + a^2} } = \sinh^{-1} {\frac x a} + C$

Proof
Let:

Also see

 * Primitive of Reciprocal of Root of Variable Squared minus Constant Squared