Maths Olympiad Prep

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Combinatorics Difficulty 5.5 AIME, harder Find the answer

7. 7 Given six points A,B,C,D,E,FA, B, C, D, E, F chosen randomly on a given circumference, these points are chosen independently and are equally likely with respect to arc length. Find the probability that triangle ABCA B C and triangle DEFD E F do not intersect (i.e., have no common points).

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

Solution

[Solution] Note that the number of cyclic permutations of six distinct points distributed on a circle is
p55=5!=120p_{5}^{5}=5!=120

And due to the symmetry of the distribution, these permutations have the same probability. The number of different permutations where triangle ABCA B C and triangle DEFD E F do not intersect is
p33p33=3!×3!=36p_{3}^{3} \cdot p_{3}^{3}=3!\times 3!=36

This is determined by the internal order of A,B,CA, B, C and the internal order of D,E,FD, E, F.
Therefore, the probability that triangle ABCA B C does not intersect with triangle DEFD E F is:
p=36120=310.p=\frac{36}{120}=\frac{3}{10} .

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.