18.3.5 Opposite Angles of a Cyclic Quadrilateral Add to a Straight Angle
Core ideaIn a quadrilateral with all four corners on the circle, each pair of opposite angles adds up to exactly 180 degrees.
Module goal. Introduce the inscribed angle, with its vertex on the circle, and establish its half relationship with the central angle plus a run of corollaries — this is the heart of the stage.