Ptolemy's Theorem


Also found in: Wikipedia.

Ptolemy's theorem

[′täl·ə·mēz ‚thir·əm]
(mathematics)
The theorem that a necessary and sufficient condition for a convex quadrilateral to be inscribed in a circle is that the sum of the products of the two pairs of opposite sides equal the product of the diagonals.

Ptolemy’s Theorem

 

the theorem of elementary geometry asserting that the product of the lengths of the diagonals of a quadrangle inscribed in a circle is equal to the sum of the products of the lengths of the opposite sides. The theorem was proved by Ptolemy in the second century.