Ptolemy's theorem
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 .