Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Find the answer

A regular hexagon PROFIT has area 1. Every minute, greedy George places the largest possible equilateral triangle that does not overlap with other already-placed triangles in the hexagon, with ties broken arbitrarily. How many triangles would George need to cover at least 90%90 \% of the hexagon's area?

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

Solution

It's not difficult to see that the first triangle must connect three non-adjacent vertices (e.g. POI), which covers area 12\frac{1}{2}, and leaves three 30-30-120 triangles of area 16\frac{1}{6} each. Then, the next three triangles cover 13\frac{1}{3} of the respective small triangle they are in, and leave six 30-30-120 triangles of area 118\frac{1}{18} each. This process continues, doubling the number of 30-30-120 triangles each round and the area of each triangle is divided by 3 each round. After 1+3+6+12+24=461+3+6+12+24=46 triangles, the remaining area is 324634=48486=881<0.1\frac{3 \cdot 2^{4}}{6 \cdot 3^{4}}=\frac{48}{486}=\frac{8}{81}<0.1, and the last triangle removed triangle has area 1486\frac{1}{486}, so this is the minimum number necessary.

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