# Ptolemy's Theorem

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Let ABCD be a quadrilateral where all the vertices lie on a circle. As long as the (green) diagonals cross, ; in words, the sum of the products of the two pairs of opposite sides equals the product of the diagonals.

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Contributed by: Jay Warendorff (March 2011)

Open content licensed under CC BY-NC-SA

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"Ptolemy's Theorem"

http://demonstrations.wolfram.com/PtolemysTheorem/

Wolfram Demonstrations Project

Published: March 7 2011