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.
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