area of a quadrilateral

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

$$K^2=(s-a)(s-b)(s-c)(s-d)-abcd{\cos }^2\left(\frac{\theta +\varphi }{2}\right)$$

where $\theta$ and $\varphi$ are of the quadrilateral.Letting $d\to 0$ we obtain Heron’s formula for the area of a triangle.