Let $a,b,c,d$ be the lengths of the sides of a quadrilateral and $K$ be its area. Let $s$ be the semiperimeter. Then
 $K^{2}=(s-a)(s-b)(s-c)(s-d)-abcd\cos^{2}\left(\frac{\theta+\phi}{2}\right)$
where $\theta$ and $\phi$ are of the quadrilateral. Letting $d\to 0$ we obtain Heron’s formula for the area of a triangle.