Expected arc covering a fixed point of the circle
Question
Three points are chosen independently and uniformly on the unit circle, cutting it into three arcs. Let be the arc that contains the fixed point . Its expected length can be written as with rational. Find .
Short answer
. Measure the gap from to the nearest point going clockwise: it exceeds only if all three points avoid an arc of length , so its mean is . The counter-clockwise gap has the same mean, so . (Size-biasing makes this arc longer than the average .)
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Open the worked solutionLessons behind this question
- Continuous Random Variables and Probability Density Functions
- Expectation, Mean, and Variance
- Conditional Expectation and Variance Revisited
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