Suppose is a triangle with incircle , and is tangent to and at and respectively. The bisectors of and intersect line at and respectively, such that and . Compute the radius of .
Solution
Let denote the measures of , respectively. We have , so is cyclic. Now implies that bisects . Since by definition bisects , we see that must lie on . Hence, . If denotes the incenter of triangle , then is perpendicular to , but since are collinear, we have that . Hence, is isoceles with . Furthermore, . Moreover, since is cyclic, is a right angle. Construct on minor such that and , and let . By the Pythagorean theorem, , so that Ptolemy applied to yields . We have . Since is a length we find . Now we have . Pythagoras applied to triangle now yields , which enables us to compute . Since the area of a triangle is also equal to its semiperimeter times its inradius, we have or .
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