# Cyclic quadrilateral (Ptolemy's theorem)

## Description

In Euclidean geometry, a cyclic quadrilateral or inscribed quadrilateral is a quadrilateral whose vertices all lie on a single circle. This circle is called the circumcircle or circumscribed circle, and the vertices are said to be concyclic. Ptolemy’s theorem expresses the product of the lengths of the two diagonals of a cyclic quadrilateral as equal to the sum of the products of opposite sides.

Related formulas## Variables

p | Diagonal of the cyclic quadrilateral (m) |

q | The other diagonal of the cyclic quadrilateral (m) |

a | Side of the cyclic quadrilateral (opposite to side c) (m) |

c | Side of the cyclic quadrilateral ( opposite to side a ) (m) |

b | Side of the cyclic quadrilateral ( opposite to side d ) (m) |

d | Side of the cyclic quadrilateral ( opposite to side b) ) (m) |