Maths Olympiad Prep

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Geometry Difficulty 5.0 AIME, harder Find the answer

A regular dodecagon P1P2P12P_{1} P_{2} \cdots P_{12} is inscribed in a unit circle with center OO. Let XX be the intersection of P1P5P_{1} P_{5} and OP2O P_{2}, and let YY be the intersection of P1P5P_{1} P_{5} and OP4O P_{4}. Let AA be the area of the region bounded by XY,XP2,YP4X Y, X P_{2}, Y P_{4}, and minor arc P2P4^\widehat{P_{2} P_{4}}. Compute 120A\lfloor 120 A\rfloor.

A number or a short expression. Spacing and $ signs are ignored.

Solution

The area of sector OP2P4O P_{2} P_{4} is one sixth the area of the circle because its angle is 6060^{\circ}. The desired area is just that of the sector subtracted by the area of equilateral triangle OXYO X Y. Note that the altitude of this triangle is the distance from OO to P1P5P_{1} P_{5}, which is 12\frac{1}{2}. Thus, the side length of the triangle is 33\frac{\sqrt{3}}{3}, implying that the area is 312\frac{\sqrt{3}}{12}. Thus, we find that A=π6312A=\frac{\pi}{6}-\frac{\sqrt{3}}{12}. Thus, 120A=20π10362.817.3120 A=20 \pi-10 \sqrt{3} \approx 62.8-17.3, which has floor 45.

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