An angle ON the circle is 1/2 the intercepted arc.
Inscribed is when the vertex is on the arc.
Corollary 1: 2 inscribed angles that intercept the same arc are congruent.
Corollary 2: An angle inscribed in a semicircle is a right angle.
Corollary 3: Opposite angles of a quadrilateral inscribed in a circle are supplementary.
Chord: 2 endpoints on the chord.
If an angle is on the circle, how much of the intercepted arc is it?