If all the vertex of the quadrilateral comes on a circle then it is said to be a cyclic quadrilateral.
Theorem 10: Any pair of opposite angles of a cyclic quadrilateral has the sum of 180º.
A + B + C + D = 360º (angle sum property of a quadrilateral)
A + C = 180°
B + D = 180º
Theorem 11: If the pair of opposite angles of a quadrilateral has sum of 180º, then the quadrilateral will be cyclic.