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.