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Date: 05/09/99 at 23:00:45
From: David Krasik

Dear Dr. Math,

I've visited several math sites on Bertrand's Paradox and I've been to
the library to look for information on it, but I haven't found
anything that has helped me to clearly understand it. My teacher
showed it to my probability and statistics class basically to confuse
us and my job is to unconfuse us. Hope you can help a little.

Thank you,
David Krasik
```

```
Date: 05/10/99 at 08:27:59
From: Doctor Anthony

Bertrand's Paradox shows that you can get 3 different answers to the
question.  What is the probability that a random chord drawn in a
circle will be longer than the side of the inscribed equilateral
triangle. The three answers are 1/4, 1/3 and 1/2 and they differ
depending on the way you define 'random chord'.

Method (1)
----------
You get a random chord by choosing a point at random in the circle and
letting this point be the midpoint of the random chord.

The distance from the centre to a side of an inscribed equilateral
triangle is  a.sin(30)  = a/2  where a = radius of the circle.

The area of the circle within this distance from the centre is
therefore     pi.(a^2/4)

So the probability that a random chord exceeds the length of the side
of the triangle is this area divided by the area of the circle pi.a^2
and it follows that the required probability is
(pi.a^2/4)/(pi.a^2) = 1/4

Method (2)
----------
You draw a chord parallel to a given line with the position of the
chord uniformly distributed along a diameter which is perpendicular to
the given line. The distance of the midpoint of a side of the
equilateral triangle from the centre of the circle is a.sin(30) = a/2.
If x = distance of mid-point of chord from the centre, then the length
of the chord is greater than the side of the triangle if x < a/2. So
the probability that a chord is greater than the side of the triangle
is the probability that -a/2 < x < a/2, and this is a/(2a) = 1/2 .

Method (3)
----------
A point is chosen at random on the circumference of the circle and a
tangent drawn at this point. Then a chord is drawn such that the angle
between the chord and tangent is uniformly distributed between 0 and
180. If this angle exceeds 60 degrees and is less than 120 degrees
then the chord is longer that the side of the equilateral triangle.
So the probability that chord exceeds the side of the triangle is

120 - 60      60
--------  =  -----  =  1/3
180        180

And so we get 3 perfectly correct but different answers to the
question. This example illustrates the very important fact that there
may be equally valid but different ways of defining 'randomness' and
in some situations it is not at all clear which is the truly 'random'
method.

- Doctor Anthony, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Probability

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