Primitive of Reciprocal of a squared minus x squared/Logarithm Form 1/size of x less than a

Theorem
Let $a \in \R_{>0}$ be a strictly positive real constant. Let $\size x < a$.

Then:
 * $\displaystyle \int \frac {\d x} {a^2 - x^2} = \dfrac 1 {2 a} \ln \size {\dfrac {a + x} {a - x} } + C$