Brahmagupta, ca 630 AD, is the giant near the beginning of the Golden Age of Indian mathematics. He wrote an astronomy text, Correctly Established Doctrine of Brahma, with two important chapters of mathematics. Computations, techniques, algebra, geometry and application problems.
One of the application problems has this beautiful result, now known as Brahmagupta’s Theorem: If a cyclic quadrilateral (a quadrilateral inscribed in a circle) has diagonals that are perpendicular to each other, then the perpendicular line drawn from the point of intersection of the diagonals to any side of the quadrilateral always ...