Inscribed angles
an angle is an inscribed angle of a circle if its vertex is on the circle and its sides are chords of the circle.
Intercepted arc
the arc that lies in the interior of an inscribed angle is its intercepted arc.
Thm. 10.9
if an angle is inscribed in a circle, then its measure is half the measure of its intercepted arc.
Thm. 10.10
if two inscribed angles of a circle intercept the same arc, then the angles are congruent.
Thm. 10.11
an angle that is inscribed in a circle is a right angle if and only if its corresponding arc is a semicircle.
Thm. 10.12
a quadrilateral can be inscribed in a circle if and only if its opposite angles are supplementary.