5 TheoremIf 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.
6 The angle on the circle is half the measure of the intercepted arc. 75°
7 TheoremThe 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.
8 It’s the average of the two intercepted arcs – (80+40)/2 = 60°
9 TheoremThe 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.