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Four points are chosen independently and at random on a circle. Find the probability that chords X1X2 and X3X4 intersect a) without calculation using a symmetry argument and b) from the definition by an integral

I'm lost here. I'm thinking of using some kind of angle argument, but not sure how to go about it.

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    @user6312 - I think the definition of a "chord" is the part of the line inside the circle, so that's redundant.2011-04-18

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