Primitive of Reciprocal of Root of x squared minus a squared

Theorem

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

Proof
Let:

Also see

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