Since is parallel to , it must be
true that . Also, the measure of
a central angle is equal to the measure of the arc that subtends
it, so the measure of arc is . Use the fact that there are
in a semicircle to find the measure of arc .
Since the measure of a central angle is equal to the measure of
the arc that subtends it, the measure of is
. Also, since and are
both radii of the circle, is an isosceles triangle. Base angles
of an isosceles triangle are congruent, so . Now use this fact,
along with the fact that the sum of the measures of the interior
angles of a triangle is , to find the measure of .