Maths Olympiad Prep

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Geometry Difficulty 4.8 AIME Find the answer Canada

Four semi-circles are arranged so that their diameters form a
66 by 88 rectangle. A circle is drawn through the four vertices of the rectangle. In the diagram, the region inside the four semi-circles but outside the circle is shaded. The total area of the shaded region is AA. What is the integer closest to AA?

Figure 0

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To calculate the shaded area, we add the area of the rectangle
and the areas of the four semi-circles, and subtract the area of the
larger circle.

Since the rectangle is 66 by 88, its area is 6×8=486 \times 8 = 48. The two semi-circles of diameter 66 together form a complete circle of diameter 66, or radius 33. The combined area of these semi-circles is π×32\pi \times 3^2 or 9π9\pi. The two semi-circles of diameter 88 together form a complete circle of diameter 88, or radius 44. The combined area of these semi-circles is π×42\pi \times 4^2 or 16π16\pi. Since the larger circle passes through the four vertices of the rectangle, the diagonal of the rectangle is its diameter. (This is because the diagonal subtends an angle of 90°90\degree at each of the other vertices and so is a diameter.) The length of the diagonal is $62\$\sqrt{6^2} +
8^2} = 100\sqrt{100} = 10, and so the radius of the larger circle is

Figure for this problem5,andsoitsareais, and so its area is π×\pi \times 5^2 = 25π25\pi. Finally, this means that the area of the shaded region is

Figure for this problem48 + 9 π\pi + 16 π\pi - 25 π\piwhichequals which equals 48.Theclosestintegerto. The closest integer to 48is is 48$.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.