Theorem 1: If an angle is inscribed in a circle, then its measure is half the measure of its intercepted arc.
m< B = 1/2
IF AND ONLY IF QS is a diameter of the circle.
< R = 90
Theorem 3: A quadrilateral can be inscribed in a circle IF AND ONLY IF its opposite angles are supplementary.
m< D + m <F = 180
m<C + m<E = 180