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Suppose that we assume (w.l.o.g.) that the first point goes in at a
point we label 0. We will cut the circle at that point and turn it
into a line segment from 0 to 1. We label points 2 and 3 by x and y,
and we draw a 1 x 1 square labeled with x and y in the usual way. Then
the following picture demonstrates the probability that the 3 points are
on a semicircle: the shaded parts represent the parts that are not on
a semicircle, so the probability is .75.
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