### Theory:

A general quadrilateral can be split into two triangles by drawing one of its diagonals.

Area of quadrilateral: The area is half the product of the diagonal and sum of heights of two triangles in it.

Area of quadrilateral = $\frac{1}{2}d\left({h}_{1}+{h}_{2}\right)$.

where $$d$$ denotes the length of diagonal.

where ${h}_{1}$ and ${h}_{2}$ is denoted as heights.

Proof: Area of quadrilateral = $\frac{1}{2}d\left({h}_{1}+{h}_{2}\right)$.

Area of quadrilateral $$ABCD =$$ (Area of $$△ ABC$$) $$+$$ (Area of  $$△ ACD$$).

$$=$$ $\left(\frac{1}{2}\phantom{\rule{0.147em}{0ex}}\mathit{AC}×{h}_{1}\right)$ $$+$$ $\left(\frac{1}{2}\phantom{\rule{0.147em}{0ex}}\mathit{AC}×{h}_{2}\right)$.

$$=$$ $\frac{1}{2}\mathit{AC}\left({h}_{1}+{h}_{2}\right)$.

$$=$$ $\frac{1}{2}d\left({h}_{1}+{h}_{2}\right)$.

Therefore, Area of quadrilateral $$=$$ $\frac{1}{2}d\left({h}_{1}+{h}_{2}\right)$ $$square units$$.