解:极角a服从均匀分布U[0, 2*pi]
设x的分布函数为F(x) = P(X <= x), -R<= x <= R
则F(x) = P(X <= x) = P(cos(a) <= x/R) = P(arccos(x/R)<=a <= 2pi - arccos(x/R))
=[2*pi- 2 *arccos(x/R)] / (2*pi) = 1- arccos(x/R)/pi
对F(x)求导得密度函数p(x) = 1/( (R^2 - x^2) ^ (1/2) * pi)
极角a服从均匀分布U[0, 2*pi]
设x的分布函数为F(x) = P(X)
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