Brahmagupta’s derivation of the area of a cyclic quadrilateral

AbstractThis paper shows that Propositions XII.21–27 of Brahmagupta’s Bra¯hmasphuṭasiddha¯nta (628 a.d.) constitute a coherent mathematical discourse leading to the expression of the area of a cyclic quadrilateral in terms of its sides. The radius of the circumcircle is determined by considering two auxiliary quadrilaterals. Observing that a cyclic quadrilateral is split by a diagonal into two triangles with the same circumcenter and the same circumradius, the result follows, using the tools available to Brahmagupta. The expression for the diagonals (XII.28) is a consequence. The shortcomings of earlier attempts at reconstructing Brahmagupta’s method are overcome by restoring the mathematical consistency of the text. This leads to a new interpretation of Brahmagupta’s terminology for quadrilaterals of different types