Forum Geometricorum, 6 (2006) 181--185.

Abstract: In this note we prove a theorem on quadrilaterals first published by the Italian mathematician Giusto Bellavitis in the 1850s, but that seems to have been overlooked since that time. Where Bellavitis used the functional equivalent of complex numbers to prove the result, we mostly rely on trigonometry. We also prove a converse of the theorem.

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