Eisso J. Atzema, On a flawed, 16th-century derivation of Brahmagupta's formula for the area of a cyclic quadrilateral,
Forum Geometricorum, 15 (2015) 165--172.
Abstract. Around 1545, the Indian commentator Ganesa suggested an interesting, but ultimately flawed, derivation of Brahmagupta's formula for the area of a cyclic quadrilateral in terms of its sides. In this paper we show that Ganesa's approach is actually valid and that his proof is easily fixed. We will also investigate to what extent his idea can be generalized to arbitrary (convex) quadrilaterals.
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