Maths Olympiad Prep

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Geometry Difficulty 6.1 National olympiad Prove it Estonia

Around each vertex of a regular hexagon of side length 3\sqrt{3} in a plane, one draws a circle of radius 11 with centre at that vertex and paints the region inside the circle blue. Find the area of the part of the plane that is painted blue.

Solution

The circles drawn around two neighbouring vertices of the hexagon intersect, since 21>32 \cdot 1 > \sqrt{3}. Hence every two neighbouring circles have a common region of the shape of a lens. Since a regular hexagon can be put together from six equilateral triangles, the circumradius of the hexagon equals 3\sqrt{3}. The altitude of one equilateral triangle is (3)2(32)2=32\sqrt{(\sqrt{3})^2 - (\frac{\sqrt{3}}{2})^2} = \frac{3}{2}.

The distance between a vertex and the second one counting from that vertex along the circumference is 33, since it equals twice the altitude of the equilateral triangle (Fig. 17). As 21<32 \cdot 1 < 3, circles drawn around such two vertices do not intersect. Thus the circles drawn around opposite vertices do not intersect either.

Figure 1
Fig. 17
Figure 2
Fig. 18

(AB2)2=AC2(\frac{AB}{2})^2 = AC^2, i.e., (CD2)2+(32)2=12(\frac{CD}{2})^2 + (\frac{\sqrt{3}}{2})^2 = 1^2, whence CD=1CD = 1, implying that the triangle ACDACD is equilateral. Thus the area of sector ACDACD and triangle ACDACD equal 16π\frac{1}{6}\pi and 34\frac{\sqrt{3}}{4}, respectively. The area of the region painted blue is 6π12(16π34)6\pi - 12(\frac{1}{6}\pi - \frac{\sqrt{3}}{4}), which equals 4π+334\pi + 3\sqrt{3}.

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