The Third Corollary to the Inscribed Angle Theorem Third corollary to the inscribed angle theorem The opposite angles of a quadrilateral inscribed in a circle are supplementary A B up to C Remember that the measures of supplementary anglesadd O D This theorem means the angles B and D are going to be and angles A and be supplementary are going to Remember thata circle has 360 and that the measure of angle is half the measure of the intercepted