Skip to main content

2012 AMC 12A Problem 14

Problem 14 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?

Answer choices

Show solution

Solution

Each arc has length 2π3\dfrac{2\pi}{3} on a unit circle, so it is a 120120^\circ sector. The nine equal sectors can be reassembled so that the enclosed region equals the regular hexagon of side 22 plus one full circle of radius 1.1. A regular hexagon of side 22 splits into 66 equilateral triangles of side 2,2, so its area is 63422=63.6 \cdot \dfrac{\sqrt3}{4} \cdot 2^2 = 6\sqrt3. Adding the unit circle’s area π\pi gives π+63.\pi + 6\sqrt3. Thus, the correct answer is E.

More practice

Concepts: area decomposition · sector · regular polygon

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