Derivative of Arccosine Function/Corollary

Corollary to Derivative of Arccosine Function
Let $a \in \R$ be a constant

Let $x \in \R$ be a real number such that $x^2 < a^2$.

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

Then:
 * $\dfrac {\mathrm d \left({\arccos \left({\frac x a}\right)}\right)} {\mathrm d x} = \dfrac {-1} {\sqrt {a^2 - x^2}}$