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I just learned about Bertrand paradox in today's class, and am very shocked. I was wondering if there are indeed only 3 (known?) unique ways of picking a chord in a circle at random, with 3 different probability values for the lengths being shorter than a side of an inscribed equilateral triangle, or whether there are infinitely many unique ways of picking a random chord in a circle of this length (and consequently infinitely many unique probabilities corresponding to each random picking method [countably/uncountably infinite then?]). Thanks!

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another way:to select a chord of random length varying from 0 to 2r. favorable selection length varies from 1.7r(length of side) to 2r. so probability~2r-1.7r/2r=3/20. – user36519 Jul 25 '12 at 16:27
up vote 5 down vote accepted

There are infinitely many different ways to randomly pick a chord from a circle. One way to construct infinitely many would be to create a probability distribution defined over the circle and pick two points accordingly, of which there are infinitely many. However, not all of these seem "natural" like the 3 in Bertrand's paradox, which is what makes it interesting.

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thanks, are there countably or uncountably infinitely many ways to pick it then? – user9466 Apr 12 '11 at 0:34
@cantlogin: there are uncountably many ways, but only countably many which can be defined in a finite number of characters from a finite alphabet. – Henry Apr 12 '11 at 1:00

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