Inradius in Terms of Circumradius

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

Let $r$ denote the inradius of $\triangle ABC$. Let $R$ denote the circumradius of $\triangle ABC$.

Then:
 * $r = 4 R \sin \dfrac A 2 \sin \dfrac B 2 \sin \dfrac C 2$