PQRS is cyclic quadrilateral. ∠P + ∠R = 180 Also in a parallelogram opposite angles will be equal. ∠P + ∠R = 180° ∠P = ∠R = 90°.
This means that PQRS must be a rectangle, otherwise, it is not cyclic.
Prove that a cyclic parallelogram is a rectangle.
Prove that the non-rectangular parallelogram is not a cyclic quadrilateral.