Let ABCD be a cyclic parallelogram
Then,
∠A = ∠C (opposite ∠s of a llgram) .........(i)
and, ∠A + ∠C = 180⁰ (opposite angles of a cyclic quadrilateral are supplementary)
or, ∠A + ∠A = 180⁰ [From (i)]
or, 2∠A = 180⁰
or, ∠A = 90⁰
Therefore, ABCD is a rectangle, since one of its angles is 90⁰