So close yet so far

Calculus Level 4

Two people stand on the perimeter of a circular field with radius 10 miles. What is the expected distance between them, in feet?

Note: 1 mile is equivalent to 5280 feet.


The answer is 67227.048.

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1 solution

Alex Li
Jun 26, 2015

Denote the angle of the minor arc between the two people as θ \theta . Note that the distance between the two is given by 10 × sin θ 2 × 2 = 20 sin θ 2 10\times\sin{\frac{\theta}{2}}\times2= 20\sin{\frac{\theta}{2}} miles. We now integrate this function from 0 0 to π \pi in order to calculate the expected value. 0 π 20 sin θ 2 d θ = 20 × 2 × ( cos ( π 2 ) + cos ( 0 ) ) = 40 \int_0^\pi 20\sin{\frac{\theta}{2}} d\theta = 20\times2\times(-\cos(\frac{\pi}{2})+\cos(0))=40 . Now, to calculate the expected distance, we divide this by π \pi , and our distance is 40 π \frac{40}{\pi} miles 67227 \approx \boxed{67227} feet.

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