Law of Tangents/Corollary

Corollary to Law of Tangents
Let $\triangle ABC$ be a triangle whose sides $a, b, c$ are such that $a$ is opposite $A$, $b$ is opposite $B$ and $c$ is opposite $C$.

Then:
 * $\tan \dfrac {A - B} 2 = \dfrac {a - b} {a + b} \cot \dfrac C 2$

Also known as
Some sources present this as the Law of Tangents itself.