5TheoremIf a tangent and a secant (or a chord) intersect ON a circle at the point of tangency, then the measure of the angle formed is one half the measure of its intercepted arc.
6The angle on the circle is half the measure of the intercepted arc. 75°
7TheoremThe measure of an angle formed by two secants or chords that intersect in the interior of a circle is one-half the sum of the measures of the arcs intercepted by the angle and its vertical angle.
8It’s the average of the two intercepted arcs – (80+40)/2 = 60°
9TheoremThe measure of an angle formed by two secants intersecting in the exterior of a circle is one-half the difference of the measures of the intercepted arcs.