Area of Triangle in Terms of Inradius

Theorem
Let $\triangle ABC$ be a triangle whose sides are $a$, $b$ and $c$ opposite vertices $A$, $B$ and $C$ respectively.

Then the area $\AA$ of $\triangle ABC$ is given by:
 * $\AA = r s$

where:
 * $r$ is the inradius of $\triangle ABC$
 * $s = \dfrac {a + b + c} 2$ is the semiperimeter of $\triangle ABC$.

Also presented as
This can also been seen in some sources presented as:
 * $r = \dfrac \AA s$