Random Chord Paradox

number of random chords

100

random seed

1

54

35

17

Bertrand's paradox asks for the length of a random chord in a unit circle. What are the odds that the length is greater than



3

? As shown in this Demonstration, the answer is

1

2

,

1

3

, or

1

4

, depending on how random is defined.

In the first case, a

y

value in (-1,1) is chosen, and a chord is drawn. Optionally, the chord can be rotated by a random angle. About half of the chords will have length greater than



3

.

In the second case, a random point on the edge of the circle is chosen, and a chord is connected to the lowest point of the circle. Optionally, both points can be chosen randomly. About a third of the chords will have length greater than



3

.

In the third case, a random point inside the circle is chosen, and this is used as the midpoint of a chord. About a quarter of the chords will have length greater than



3

.