Primitive of Composite Function

Theorem
Let $f$ and $g$ be a real functions which are integrable.

Let the composite function $g \circ f$ also be integrable.

Then:


 * $\displaystyle \int \map {g \circ f} x \rd x = \int \frac {\map g u} {\map {f'} x} \rd u$

where $u = \map f x$.