Eisso J. Atzema, A theorem by Giusto Bellavits on a class of
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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