In figure, , where P, Q and R are points on a circle with centre O. Find .
P, Q and R are the points on a circle with centre O where
Construction: S is a point on the major arc PR. Join P and S, R and S to form a cyclic quadrilateral.
(Sum of opposite angles of a cyclic quadrilateral is equal to )
(Angle subtended by an arc at the centre is double the angle subtended by it at the circumference of the circle.)
In , we have
But, we have (Angles opposite to the equal sides in a triangle are equal.)