Let ABC be a triangle with AB=3,BC=4, and CA=5. Let A1,A2 be points on side BC, B1,B2 be points on side CA, and C1,C2 be points on side AB. Suppose that there exists a point P such that PA1A2,PB1B2, and PC1C2 are congruent equilateral triangles. Find the area of convex hexagon A1A2B1B2C1C2.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Since P is the shared vertex between the three equilateral triangles, we note that P is the incenter of ABC 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 32. Furthermore, since the incenter is the intersection of angle bisectors, it is easy to see that AB2=AC1,BC2=BA1, and CA2=CB1. Using the fact that the altitudes from P to AB and CB form a square with the sides, we use the side lengths of the equilateral triangle to compute that AB2=AC1=2−31,BA1=BC2=1−31, and CB1=CA2=3−31. We have that the area of the hexagon is therefore 6−(21(2−31)2⋅54+21(1−31)2+21(3−31)2⋅53)=1512+223
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