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Geometry Difficulty 5.5 AIME, harder Find the answer

Let ABCA B C be a triangle with AB=3,BC=4A B=3, B C=4, and CA=5C A=5. Let A1,A2A_{1}, A_{2} be points on side BCB C, B1,B2B_{1}, B_{2} be points on side CAC A, and C1,C2C_{1}, C_{2} be points on side ABA B. Suppose that there exists a point PP such that PA1A2,PB1B2P A_{1} A_{2}, P B_{1} B_{2}, and PC1C2P C_{1} C_{2} are congruent equilateral triangles. Find the area of convex hexagon A1A2B1B2C1C2A_{1} A_{2} B_{1} B_{2} C_{1} C_{2}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Since PP is the shared vertex between the three equilateral triangles, we note that PP is the incenter of ABCA B C since it is equidistant to all three sides. Since the area is 6 and the semiperimeter is also 6, we can calculate the inradius, i.e. the altitude, as 1, which in turn implies that the side length of the equilateral triangle is 23\frac{2}{\sqrt{3}}. Furthermore, since the incenter is the intersection of angle bisectors, it is easy to see that AB2=AC1,BC2=BA1A B_{2}=A C_{1}, B C_{2}=B A_{1}, and CA2=CB1C A_{2}=C B_{1}. Using the fact that the altitudes from PP to ABA B and CBC B form a square with the sides, we use the side lengths of the equilateral triangle to compute that AB2=AC1=213,BA1=BC2=113A B_{2}=A C_{1}=2-\frac{1}{\sqrt{3}}, B A_{1}=B C_{2}=1-\frac{1}{\sqrt{3}}, and CB1=CA2=313C B_{1}=C A_{2}=3-\frac{1}{\sqrt{3}}. We have that the area of the hexagon is therefore 6(12(213)245+12(113)2+12(313)235)=12+223156-\left(\frac{1}{2}\left(2-\frac{1}{\sqrt{3}}\right)^{2} \cdot \frac{4}{5}+\frac{1}{2}\left(1-\frac{1}{\sqrt{3}}\right)^{2}+\frac{1}{2}\left(3-\frac{1}{\sqrt{3}}\right)^{2} \cdot \frac{3}{5}\right)=\frac{12+22 \sqrt{3}}{15}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.