Ptolemy's theorem
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English[edit]
Etymology[edit]
Named after the Greek astronomer and mathematician Ptolemy.
Proper noun[edit]
- (geometry) A theorem that states in a cyclic quadrilateral, the product of the lengths of its diagonals is equal to the sum of the products of the lengths of the two pairs of opposite sides.
Usage notes[edit]
In mathematical terms, the theorem states if the vertices of a cyclic quadrilateral are A, B, C, and D in order, then .