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# The elusive chord

21 replies to this topic

### #11 Prof. Templeton

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Posted 16 November 2012 - 02:22 PM

I agree with CaptainEd. Jaynes' solution to the Bertrand paradox shows that random radius (method 2) is the only robust theoretical means of choosing random chord.

So, then it is more random. I looked up Jaynes' proposal and it based on the fact that the size of the circle is unknown, here the OP has given us a size. Is throwing straws at a circle a better method than selecting a number at random between 0 and πr2 , letting that number represent an area and drawing a chord to section off that area?
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### #12 bonanova

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Posted 16 November 2012 - 04:37 PM

Indeed, more than one way.

Bonanova, what is the goal of this puzzle?
a) show us that the notion of "randomly chosen" depends on how you define it?
b) have us show lots of ingenuity in defining truly different ways to define it?
c) elicit from us the really-really-best way to define it?
d) all of the above
e) none of the above
f) other

All of the above, probably.

Let's make it a competition to show the greatest and least among any reasonable definition of randomness.

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### #13 bonanova

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Posted 20 November 2012 - 03:28 PM

Well since I neglected to use the word "average" in the OP, another answer is anything between 0 and 2.
That was the "fourth" answer.
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### #14 joef1000

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Posted 22 November 2012 - 04:38 PM

Spoiler for

Edited by joef1000, 22 November 2012 - 04:40 PM.

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### #15 joef1000

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Posted 22 November 2012 - 04:52 PM

My above solution assumes that you define random in a way that makes all chord lengths equally likely.
(I hate the edit system on this site)

Edited by joef1000, 22 November 2012 - 04:52 PM.

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### #16 bonanova

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Posted 22 November 2012 - 06:16 PM

Spoiler for

This is an interesting approach. The radius is a middle measure of the extremes of zero and the diameter, but It is the median, not the mean. One must represent (find an expression for) all the uncountably infinite chords and then examine the distribution of their lengths. If the distribution were found to be uniform from 0 to 2r then r would be correct. All the examples given previously in this thread, however, show the lengths not to be uniformly distributed.
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### #17 bonanova

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Posted 22 November 2012 - 06:20 PM

My above solution assumes that you define random in a way that makes all chord lengths equally likely.
(I hate the edit system on this site)

I'm not sure you can define random in any sense other than an unbiased selection from all available instances. The challenge that arises is that several exhaustive, random [in some sense] and uncountable subsets each yield a distinct value of average length.
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### #18 Prof. Templeton

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Posted 27 November 2012 - 03:36 PM

After doing some research and none of the math, I've found
Spoiler for that

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### #19 bonanova

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Posted 27 November 2012 - 05:39 PM

After doing some research and none of the math, I've found

Spoiler for that

Inscribe a regular polygon of n sides where n > 1.
Take the chord to be one side of the polygon; (n = 2 gives a diameter.)
Let n be chosen at random.

What is that average chord length?
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### #20 Prof. Templeton

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Posted 27 November 2012 - 08:27 PM

Inscribe a regular polygon of n sides where n > 1.
Take the chord to be one side of the polygon; (n = 2 gives a diameter.)
Let n be chosen at random.

What is that average chord length?

Quite small. The larger n is the smaller the chord length. I would argue that this is not an acceptable way to chose "random chords", however.
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