Ptolemy's theorem

Relates the 4 sides and 2 diagonals of a quadrilateral with vertices on a common circle

In Euclidean geometry, Ptolemy's theorem is a relation between the four sides and two diagonals of a cyclic quadrilateral. The theorem is named after the Greek astronomer and mathematician Ptolemy. Ptolemy used the theorem as an aid to creating his table of chords, a trigonometric table that he applied to astronomy.

This keyword could refer to multiple things. Here are some suggestions: