Egyptian Formula for Area of Quadrilateral

Theorem
Let $\Box ABCD$ be a quadrilateral.

Let the sides of $\Box ABCD$ be $a$, $b$, $c$ and $d$ such that $a$ is opposite $c$ and $b$ is opposite $d$.

Then the area of $\Box ABCD$ can be approximated by:


 * $\map \Area {\Box ABCD} \approx \dfrac {a + c} 2 \times \dfrac {b + d} 2$

The closer $\Box ABCD$ is to a rectangle, the better the approximation.

Also known as
This formula can also be seen as the Roman-Egyptian, Egyptian-Roman or Roman formula.