Maths Olympiad Prep

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, 2024

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?

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 55, and so its area is π×52=25π\pi \times 5^2 = 25\pi.

Finally, this means that the area of the shaded region is 48+9π+16π25π48 + 9 \pi + 16 \pi - 25 \pi which equals
4848.

The closest integer to 4848 is 4848.

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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.