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jmc

geometry senior

Problem

The two squares shown share the same center and have sides of length 1. The length of is and the area of octagon is where and are relatively prime positive integers. Find
Solution
Triangles , , , etc. are congruent by symmetry (you can prove it rigorously by using the power of a point to argue that exactly two chords of length in the circumcircle of the squares pass through , etc.), and each area is . Since the area of a triangle is , the area of all of them is and the answer is .
Final answer
185