# Cyclic quadrilateral (cosine of an angle)

## 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. A convex quadrilateral ABCD is cyclic if and only if its opposite angles are supplementary. The cosine of an angle of a cyclic quadrilateral can be calculated by the sides of the quadrilateral.

Related formulas## Variables

A | Angle of the quadrilateral between sides AB and AD (degrees) |

a | Length of the side of the cyclic quadrilateral (AB) (m) |

d | Length of the side of the cyclic quadrilateral (DA) (m) |

b | Length of the side of the cyclic quadrilateral (BC) (m) |

c | Length of the side of the cyclic quadrilateral (CD) (m) |