u/Fun-Fan-4741

I guessed the following area formula for a (planar) quadrilateral using two opposite sides and four angles:(When $a=AB$ and $c=CD$,)

\[S =\frac{a^2}{2(\cot A + \cot B )} + \frac{c^2}{2(\cot C + \cot D)} \]

This formula can be applied to the quadrilateral satisfying $A+B \ne \pi$ and $C+D \ne \pi$, including convex, concave, and even self-intersecting cases.

I'd like to know the applicabile case , inapplicabile case and reasons for the validity of those cases. Are there any known papers or more elegant ways to derive this formula?I guess there are many ways to explain this formula. It greatly resembles $S = \frac{1}{2}(ab \sin B + cd \sin D)$. It seems that $b$ and $d$ can be determined using $a, c, A, B, C, D$.