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2012 AMC 10A Problem 18

Problem 18 of 25IntermediateGeometry

The closed curve in the figure is made up of 99 congruent circular arcs each of length 2π3,\dfrac{2\pi}{3}, where each of the centers of the corresponding circles is among the vertices of a regular hexagon of side 2.2. What is the area enclosed by the curve?

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Solution

Each arc has radius 11 and length 2π3,\frac{2\pi}{3}, so the corresponding circular sectors all have the same area. As the diagram shows, the sector pieces may be rearranged: everything added to the regular hexagon combines to exactly one unit circle. The regular hexagon is made of six equilateral triangles of side 2,2, so its area is 6(3422)=63.6\left(\dfrac{\sqrt3}{4}\cdot2^2\right)=6\sqrt3. Adding the unit circle gives a total enclosed area of π+63.\pi+6\sqrt3. Thus, E is the correct answer.

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Concepts: sector · regular polygon · area decomposition

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.