Four semi-circles are arranged so that their diameters form a 6 by 8 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 A. What is the integer closest to A?
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 6 by 8, its area is 6×8=48. The two semi-circles of diameter 6 together form a complete circle of diameter 6, or radius 3. The combined area of these semi-circles is π×32 or 9π. The two semi-circles of diameter 8 together form a complete circle of diameter 8, or radius 4. The combined area of these semi-circles is π×42 or 16π. 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° at each of the other vertices and so is a diameter.) The length of the diagonal is $62 + 8^2} = 100 = 10, and so the radius of the larger circle is
5,andsoitsareaisπ× 5^2 = 25π. Finally, this means that the area of the shaded region is
Want a route through all this instead of an archive? The track
puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.