Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Prove it United States

Problem:
Two points are selected independently and uniformly at random inside a regular hexagon. Compute the probability that a line passing through both of the points intersects a pair of opposite edges of the hexagon.

Solution

Solution:
Figure 1
First, we compute the probability that the line through two random points in a triangle ABCABC passes through segments AB\overline{AB} and AC\overline{AC}. We can take an affine transform of the two random points and the triangle such that ABCABC becomes equilateral. Since the distribution of the two points is still uniform and independent, the probability of the line intersecting any two given sides is 13\frac{1}{3} by symmetry.

Next, we compute the probability that the line through two random points in a rectangle ABCDABCD passes through opposite edges AB\overline{AB} and CD\overline{CD}.
Figure 2
If the line passes through AB\overline{AB} and BC\overline{BC}, the points must both lie in triangle ABCABC, whose area is half that of ABCDABCD. Given this, the probability the line passes through those two sides is 13\frac{1}{3}, as computed before. Thus the probability the line passes through AB\overline{AB} and BC\overline{BC} is (12)213=112\left(\frac{1}{2}\right)^2 \cdot \frac{1}{3} = \frac{1}{12}. The same goes for the other pairs of adjacent edges. By symmetry, the line is equally likely to pass through either pair of opposite edges, each with probability 12(14112)=13\frac{1}{2}\left(1 - 4 \cdot \frac{1}{12}\right) = \frac{1}{3}.

Figure 3
We now return to the original problem. If the line passes through a pair of opposite edges, then both points must be in the rectangle formed by these edges, which has area 23\frac{2}{3} that of the hexagon. Given this, the probability the line passes through those two edges is 13\frac{1}{3} as computed before. Thus, the probability that the line passes through the given pair of opposite edges is (23)213=427\left(\frac{2}{3}\right)^2 \cdot \frac{1}{3} = \frac{4}{27}. Hence, the probability the line passes through any of the three pairs of opposite edges is 3427=[49]3 \cdot \frac{4}{27} = \left[\frac{4}{9}\right].

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.