Brahmagupta's formula finds the area of any cyclic convex quadrilateral (one that can be inscribed in a circle) given the lengths of the sides.
This formula generalizes Heron's formula for the area of a triangle. A triangle may be regarded as a quadrilateral with one side of length zero. From this perspective, as d approaches zero, a cyclic quadrilateral converges into a cyclic triangle (all triangles are cyclic), and Brahmagupta's formula simplifies to Heron's formula.
[url]http://en.wikipedia.org/wiki/Brahmagupta%27s_formula[/url]
