Therefore, the probability distribution function f(x, y) =1/s =1/(π R2) x2+y2.
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e(z)=∫zf(z)dz=∫(x^2+y^2)^0.5/(πr^2)dxdy=∫dθ(0~2π)∫r^2/(πr^2)dr(0~r)=2r/3