In geometry, a

**cyclic quadrilateral**is a quadrilateral whose vertices are all lie on a single circle. The vertices are said to be

*concyclic*.

- Opposite angles of a cyclic quadrilateral add up to .
- The area of a cyclic quadrilateral is given by Brahmagupta's formula.
- The area of a cyclic quadrilateral is maximal among all quadrilaterals having the same (multi)set of side lengths.