Answer: Opposite angles of a rectangle are \(90^\circ + 90^\circ = 180^\circ\), so by the converse of the cyclic quadrilateral theorem it is cyclic.
Let □ABCD be a rectangle. Each of its angles is \(90^\circ\).
\[\angle A + \angle C = 90^\circ + 90^\circ = 180^\circ\]A pair of opposite angles is supplementary, so by the converse of the cyclic quadrilateral theorem, □ABCD is cyclic.
(Also: \(\angle ABC = \angle ADC = 90^\circ\), so B and D lie on the circle with diameter AC.)