Problem:
Let be a quadrilateral with an inscribed circle centered at . Let intersect at . If , , and , then what are all possible measures of ?
Solution
Solution:
Arbitrarily defining and determines and up to reflections across . lies on both the circle determined by and the line through tangent to the circle (and on the opposite side of ); since the intersection of a line and a circle has at most two points, there are only two cases for . The diagram below on the left shows the construction made in this solution, containing both cases. The diagram below on the right shows only the degenerate case.

Reflect across to , then . Since and are tangent to the circle centered at , is the angle bisector of . Then . If , then . Otherwise, since (given), is a cyclic quadrilateral. Then and , so .
Since is exterior to , . Then . Because is cyclic, . Then .
Thus, the two possible measures are and .