Question 419596
I don't recall the formula {{{A = sqrt(abcd)}}} (maybe I just haven't seen it. Note that the quadrilateral must be able to have circles inscribed and circumscribed around it). I am pretty familiar with Brahmagupta's formula, which is a special case of Bretschneider's formula (the area of any quadrilateral with known interior angles).


The proof is somewhat lengthy, so I won't post one here. However, Wikipedia has some good articles about Bretschneider's and Brahmagupta's formulas, including their proofs, so I'll post them here:


http://en.wikipedia.org/wiki/Bretschneider's_formula
http://en.wikipedia.org/wiki/Brahmagupta%27s_formula