2003 AMC 12B Problems/Problem 25
Three points are chosen randomly and independently on a circle. What is the probability that all three pairwise distance between the points are less than the radius of the circle?
The first point is placed anywhere on the circle, because it doesn't matter where it is chosen.
The next point must lie within degrees of arc on either side, a total of degrees possible, giving a total chance. The last point must lie within degrees of both points.
The minimum area of freedom we have to place the third point is a degrees arc(if the first two are degrees apart), with a probability. The maximum amount of freedom we have to place the third point is a degree arc(if the first two are the same point), with a probability.
As the second point moves farther away from the first point, up to a maximum of degrees, the probability changes linearly (every degree it moves, adds one degree to where the third could be).
Therefore, we can average probabilities at each end to find , the average probability we can place the third point based on a varying second point.
Therefore the total probability is or
We will use geometric probability.
The first point can be anywhere. Each point must be or less away from each other.
Define be the amount of radians away the second point is from the first. We limit to be in the interval . Define be the amount of radians away the third point is from the first. We limit to be in the interval . Now, we can deduct that: and We now begin plotting these on the coordinate grid.
First note that the area of the points that and can be (ignoring the conditions) is (remember what we restricted and to).
Now, we can graph the equations we deduced on the coordinate grid. That should look like this: The area of the shaded region can be calculated in many ways. Eventually, you will find that the area is .
Thus, the probability is , or .
There are different ways to choose points out of . The probability that an angle between points is smaller than is . The probability that the third point is inside the angle is
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