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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 AA be the arc that contains the fixed point (1,0)(1,0). Its expected length can be written as qπq\pi with qq rational. Find qq.

Short answer

q=1q=1. Measure the gap from (1,0)(1,0) to the nearest point going clockwise: it exceeds xx only if all three points avoid an arc of length xx, so its mean is ∫02π(1−x2π)3dx=π2\int_0^{2\pi}(1-\tfrac{x}{2\pi})^3dx=\tfrac{\pi}{2}. The counter-clockwise gap has the same mean, so E[length]=π\mathbb{E}[\text{length}]=\pi. (Size-biasing makes this arc longer than the average 2π/32\pi/3.)

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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