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Printjmc
geometry senior
Problem
In the adjoining figure, is the diameter of a semicircle with center . Point lies on the extension of past ; point lies on the semicircle, and is the point of intersection (distinct from ) of line segment with the semicircle. If length equals length , and the measure of is , then find the measure of , in degrees.

Solution
Draw . Let . Since , triangle is isosceles, so . Angle is exterior to triangle , so .
Triangle is isosceles, so . Then is external to triangle , so . But , so .
Triangle is isosceles, so . Then is external to triangle , so . But , so .
Final answer
15^\circ