Inradius in Terms of Circumradius

Theorem
Let $\triangle ABC$ be a triangle whose sides are of lengths $a, b, c$.

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

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