Suppose ABCD is a rectangle.
∴ ∠A = ∠B = ∠C = ∠D = 90º (Each angle of a rectangle is 90º)
⇒ ∠A + ∠C = 180º and ∠B + ∠D = 180º
We know, if a pair of opposite angles of a quadrilateral is supplementary, then quadrilateral is cyclic.
∴ Rectangle ABCD is a cyclic quadrilateral.
So, any rectangle is a cyclic quadrilateral.