Cyclic polygon

In plane geometry, a cyclic polygon is a polygon whose vertices all lie on one circle. The centre of the circle is its circumcentre,

Every triangle is cyclic, since any three (non-collinear) points lie on a unique circle.

A cyclic quadrilateral is a quadrilateral whose foue vertices are concyclic. A quadrilateral is cyclic if and only iff pairs of opposire angles are supplementary (add up to 180°, π radians).