Derivative of Arctangent Function/Corollary

Corollary to Derivative of Arctangent Function
Let $x \in \R$.

Let $\arctan \left({\dfrac x a}\right)$ be the arctangent of $\dfrac x a$.

Then:
 * $\dfrac {\mathrm d \left({\arctan \left({\frac x a}\right) }\right)} {\mathrm d x} = \dfrac a {a^2 + x^2}$