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Ptolemy's theorem[′täl·ə·mēz ‚thir·əm]
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
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.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.