Primitive of Reciprocal of x by Root of x squared plus a squared/Logarithm Form

Theorem

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