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2003 AMC 12B Problem 16

Problem 16 of 25IntermediateGeometry

Three semicircles of radius 11 are constructed on diameter AB\overline{AB} of a semicircle of radius 2.2. The centers of the small semicircles divide AB\overline{AB} into four line segments of equal length, as shown. What is the area of the shaded region that lies within the large semicircle but outside the smaller semicircles?

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Solution

The large semicircle has area 12π(2)2=2π.\dfrac{1}{2}\pi(2)^2 = 2\pi. The three small semicircles have total area 3π2\dfrac{3\pi}{2} before their overlaps are accounted for. The centers of each adjacent pair are 11 unit apart, so their intersection is bounded by two 6060^\circ sectors and an equilateral triangle. Each of the two overlaps therefore has area π334.\dfrac{\pi}{3} - \dfrac{\sqrt{3}}{4}. The shaded area is 2π[3π22(π334)]=76π32. \begin{aligned} &2\pi - \left[\frac{3\pi}{2} - 2\left(\frac{\pi}{3} - \frac{\sqrt{3}}{4}\right)\right] \\ &= \frac{7}{6}\pi - \frac{\sqrt{3}}{2}. \end{aligned} Thus, the correct answer is E.

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Concepts: sector · equilateral triangle · area decomposition

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