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:


 * $\displaystyle \int g \circ f \left({x}\right) \ \mathrm d x = \int \frac {g \left({u}\right)}{f' \left({x}\right)} \ \mathrm d u$

where $u = f \left({x}\right)$.