Exradius of Triangle in Terms of Circumradius

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

Let $\rho_a$ be the exradius of $\triangle ABC$ the excircle which is tangent to $a$.

Let $R$ be the circumradius of $\triangle ABC$.

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