Derivative of Composite Function/Corollary

Theorem
Let $f, g, h$ be continuous real functions such that:
 * $y = \map f u, x = \map g u$

Then:
 * $\dfrac {\d y} {\d x} = \dfrac {\paren {\dfrac {\d y} {\d u} } } {\paren {\dfrac {\d x} {\d u} } }$

for $\dfrac {\d x} {\d u} \ne 0$.