What is the probability of a chord inside a circle being greater than D?

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Discussion Overview

The discussion revolves around the probability of a chord inside a circle (with radius 1) being greater than a specified length D, where D is in the interval [0, 2]. Participants explore different methods of defining chords and how these definitions impact the probability calculation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a formula for the probability as \(\frac{\pi - 2\sin^{-1}(D/2)}{\pi}\) and questions its validity.
  • Another participant argues that since the maximum chord length in a circle of radius 1 is 2, the probability of a chord being greater than 2 is 0.
  • A participant clarifies that the question pertains to specific values of D where \(0 < D < 2\) and notes that the probability will depend on the distribution function used to define the placement of the chord.
  • Two potential distribution methods are mentioned: uniform distance from the center and picking a point on the circumference with the other endpoint uniformly around the circumference.
  • Another participant expresses confusion and requests further elaboration on how the two distributions affect the probability calculation.
  • A later reply acknowledges a misreading of the question and seeks clarification on the assumptions made in the initial probability calculation.
  • One participant presents calculated probabilities for various distribution cases, indicating that different assumptions lead to different results, emphasizing that there is no singular "right" answer.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the probability calculation, as multiple competing views and methods for defining chords are presented. The discussion remains unresolved regarding the correct approach to determining the probability.

Contextual Notes

Participants highlight the importance of assumptions regarding the distribution of chords, which affects the resulting probabilities. The discussion includes various interpretations and potential methods without resolving the implications of these differences.

murshid_islam
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that day my friend asked me a question. what is the probability that the chord inside a circle (with radius 1) will be greater than D, where D is in the interval [0, 2].

i have came up with the answer
[tex]{\pi - 2\sin^{-1}(D/2)} \over {\pi}[/tex]

is it ok? or did i do anything wrong?
 
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Am I missing something? A circle of radius 1 has diameter of length 2 and all other chords are shorter. It is impossible for a chord to have length greater than 2. The probability is 0.
 
Am I missing something? A circle of radius 1 has diameter of length 2 and all other chords are shorter. It is impossible for a chord to have length greater than 2. The probability is 0.
If you put D=2 in the expression, you will get 0. The question is what is the probability for a specific D, where 0<D<2. The answer will depend on what sort of distribution function defines the placing of the chord - there is more than 1 way to do it.
Examples:
(1) Uniform in distance from the center.
(2) Pick a point on the circumference, other end point is uniform around the circumference.
 
how will the 2 distributions affect the answer? can you be a bit more elaborate?
 
Ah, yes, I simply misread the quesition!
 
how will the 2 distributions affect the answer? can you be a bit more elaborate?
You had to make some sort of assumption about the chords to get the answer you did. I haven't worked out what the rersults would be for the 2 examples I gave, but I can make up possibilities which I know would give different results, although they might look strange. For example, uniform in the square of the distance from the center.
 
I took the time to work out the various possibilities I mentioned. To simplify notation, let s=D/2. The probabiliites for these cases are:

Case..........Prob.
uniform in arc length.......murshid islam result
uniform in distance from center.....(1-s2)1/2
uniform in distance squared......1-s2
uniform in chord length.....1-s

As you can see, there is no "right" answer.
 

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