A circumscribed angle is an angle that has its vertex on the circumference of a circle and whose sides are chords of the circle. The measure of a circumscribed angle is equal to half the measure of the intercepted arc. Circumscribed angles are important in geometry because they are used to prove theorems about angles, chords, and arcs in circles. The opposite angles of a cyclic quadrilateral are always supplementary, and the sum of the angles in a cyclic polygon is always equal to (n-2)180 degrees, where n is the number of sides of the polygon.