GeometryDifficulty 5.3AIME, harderProve itUnited States
Problem:
Triangle ABC is an equilateral triangle with side length 1. Let X0,X1,… be an infinite sequence of points such that the following conditions hold:
- X0 is the center of ABC. - For all i≥0, X2i+1 lies on segment AB and X2i+2 lies on segment AC. - For all i≥0, ∡XiXi+1Xi+2=90∘. - For all i≥1, Xi+2 lies in triangle AXiXi+1.
Find the maximum possible value of ∑i=0∞∣XiXi+1∣, where ∣PQ∣ is the length of line segment PQ.
Solution
Solution:
36
Let Y be the foot of the perpendicular from A to X0X1: note that the sum we wish to maximize is simply X0Y+YA. However, it is not difficult to check (for example, by AM-GM) that AY+YX0≥2⋅AX0=36. This may be achieved by making ∠YX0A=45∘, so that ∠AX1X0=105∘.
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Source: MathNet,
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