Proving the First Case of the Inscribed Angle Theorem Given AB is the diameter of a circle and is one side of the angle that intercepts AC Prove mzABC is half the measure of AC 180 2x B You can draw line segment CD by the unique Since the triangle isl x D Triangle BCD is isosceles since BD and CD are both circle and all radii of a given circle are congruent EFO x A postulate of the its base angles are congruent