Take any four numbers a, b, c, d. Halve their sum — call it s. Subtract each from s in turn, multiply the four remainders together, and take the square root. That number is the exact area of a quadrilateral with those four sides — provided the four corners lie on a single circle. Brahmagupta wrote this down in 628 CE in his Brāhmasphuṭasiddhānta. It's Heron's triangle-area formula, extended by one factor. The European tradition rediscovered it independently via Carl Strehlke in 1842.