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Random points on a circle
Another homework question for you brilliant people out there. I got as far as I could and now I'm stuck. Any help is greatly appreciated!
Assume that you have a circle with a point P designated on the perimeter. You choose two points X and Y along the perimeter. Let XY_bar represent the length of the arc from X and Y (drawn clockwise from X) and let YX_bar represent the length of the arc from Y to X (drawn clockwise from Y). Suppose the radius of the circle is r. a) What are the marginal distributions of XY_bar and YX_bar? b) What is the correlation between XY_bar and YX_bar? c) What is the distribution of the length of the arc that covers the point P? Part a: XY_bar ~ U(0, 2*pi*r) so... Fxy_bar(xy_bar) = 0 for xy_bar < 0 xy_bar/(2*pi*r) for 0 < xy_bar < 2*pi*r 1 for xy_bar > 2*pi*r And then the same thing for YX_bar. How am I doing so far? Part b: Corr(xy_bar, yx_bar) = Cov(xy_bar, yx_bar) / [sqrt(Var(xy_bar) * sqrt(Var(yx_bar)] so Cov(xy_bar, yx_bar) = E[(XY_bar - E(XY_bar))(YX_bar - E(YX_bar))] = E[XY_bar * YX_bar] - E(XY_bar)E(YX_bar) so... E(XY_bar) = E(YX_bar) = pi*r, right? But now what is E(XY_bar * YX_bar) ??? And then for the Variances, I know that Var(X) = E(X^2) - mu^2 but I'm not sure how to use that here. Part c: I'm not really sure how to approach this part. I know that it is more likely to be in the longer segment, due to length-biased sampling. But I'm not sure how that affects the distribution. Many many thanks in advance. |
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