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Harvard-MIT Mathematics Tournament

Geometry Difficulty 4.9 AIME Prove it United States

Problem:
A circle of radius tt is tangent to the hypotenuse, the incircle, and one leg of an isosceles right triangle with inradius r=1+sin⁡π8r=1+\sin \frac{\pi}{8}. Find rtr t.

Solution

Solution:
The distance between the point of tangency of the two circles and the nearest vertex of the triangle is seen to be both r(csc⁡π8−1)r\left(\csc \frac{\pi}{8}-1\right) and t(csc⁡π8+1)t\left(\csc \frac{\pi}{8}+1\right), so
rt=r2(csc⁡π8−1)csc⁡π8+1=(1+sin⁡π8)2(1−sin⁡π8)1+sin⁡π8=1−sin⁡2π8=12+1−2sin⁡2π82=12+cos⁡π42=12+24=2+24. \begin{aligned} r t & =\frac{r^{2}\left(\csc \frac{\pi}{8}-1\right)}{\csc \frac{\pi}{8}+1}=\frac{\left(1+\sin \frac{\pi}{8}\right)^{2}\left(1-\sin \frac{\pi}{8}\right)}{1+\sin \frac{\pi}{8}}=1-\sin ^{2} \frac{\pi}{8} \\ & =\frac{1}{2}+\frac{1-2 \sin ^{2} \frac{\pi}{8}}{2}=\frac{1}{2}+\frac{\cos \frac{\pi}{4}}{2}=\frac{1}{2}+\frac{\sqrt{2}}{4}=\frac{2+\sqrt{2}}{4} . \end{aligned}

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