Primitive of Reciprocal of Root of x squared plus a squared/Mistake

Source Work

 * Appendix $7$: Integrals
 * Appendix $7$: Integrals

Mistake

 * $\ds \int \frac {\d x} {\sqrt {a^2 + x^2} } = \dfrac 1 a \tan^{-1} {\frac x a}$

Correction
This should read:


 * $\ds \int \frac {\d x} {a^2 + x^2} = \dfrac 1 a \tan^{-1} {\frac x a}$