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geometry senior
Problem
Semicircle has diameter of length . Circle lies tangent to at a point and intersects at points and . If and , then the area of equals , where and are relatively prime positive integers, and is a positive integer not divisible by the square of any prime. What is ?
(A)
(B)
(C)
(D)
Solution
Let be the center of the semicircle and be the center of the circle. Applying the Extended Law of Sines to we find the radius of Alternatively, by the Inscribed Angle Theorem, is a triangle with base Dividing into two congruent triangles, we get that the radius of is by the side-length ratios. Let be the midpoint of By the Perpendicular Chord Bisector Converse, we have and Together, points and must be collinear. By the SAS Congruence, we have both of which are triangles. By the side-length ratios, we obtain and By the Pythagorean Theorem on right we get and By the Pythagorean Theorem on right we get Let be the foot of the perpendicular from to and be the foot of the perpendicular from to as shown below: Clearly, quadrilateral is a rectangle. Since by alternate interior angles, we have by the AA Similarity, with the ratio of similitude Therefore, we get and The area of is from which the answer is
Final answer
D