A regular dodecagon P1P2⋯P12 is inscribed in a unit circle with center O. Let X be the intersection of P1P5 and OP2, and let Y be the intersection of P1P5 and OP4. Let A be the area of the region bounded by XY,XP2,YP4, and minor arc P2P4. Compute ⌊120A⌋.
A number or a short expression. Spacing and $ signs are ignored.
Solution
The area of sector OP2P4 is one sixth the area of the circle because its angle is 60∘. The desired area is just that of the sector subtracted by the area of equilateral triangle OXY. Note that the altitude of this triangle is the distance from O to P1P5, which is 21. Thus, the side length of the triangle is 33, implying that the area is 123. Thus, we find that A=6π−123. Thus, 120A=20π−103≈62.8−17.3, which has floor 45.
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