cyclic quadrilateralCyclicQuadrilateral2001-10-06 18:14:572002-03-09 01:30:11Definition\usepackage{amssymb}
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A quadrilateral is cyclic when its four vertices lie on a circle.
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A necessary and sufficient condition for a quadrilateral to be cyclic, is that the sum of a pair of opposite angles be equal to $180^\circ$.
One of the main results about these quadrilaterals is Ptolemy's theorem.
Also, from all the quadrilaterals with given sides $p,q,r,s$, the one that is cyclic has the greatest area. If the four sides of a cyclic quadrilateral are known, the area can be found using Brahmagupta's formula