KöMaL 2005 B3805

The lengths of the sides of a cyclic quadrilateral are $a$, $b$, $c$, $d$, respectively, its perimeter is 2$s$ and the measure of the angle of the sides $a$ and $b$ is $\alpha$. Prove that $\tan^2\frac{\alpha}{2}=\frac{(s-a)(s-b)}{(s-c)(s-d)}.$