7. 7 Given six points 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 and triangle do not intersect (i.e., have no common points).
Solution
[Solution] Note that the number of cyclic permutations of six distinct points distributed on a circle is
And due to the symmetry of the distribution, these permutations have the same probability. The number of different permutations where triangle and triangle do not intersect is
This is determined by the internal order of and the internal order of .
Therefore, the probability that triangle does not intersect with triangle is:
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