可能你想写：求 z=(x^2＋y^2)^0.5的密度函数.
F(z)=P(Z<=z)=P{(X^2+Y^2)^0.5<=z}
当z<0时，F(z)=0
当z>=0时，F(z)=P{X^2+Y^2<=z^2}
F(z)=P{X^2+Y^2<=z^2}=(2πσ^2)^(-1)∫∫e^[-(x^2+y^2)/(2σ^2)]dxdy,
积分区域是X^2+Y^2<=z^2
积分得概率分布：F(z)=1-e^[-z^2/(2σ^2)]，z>0
求导得概率密度：f(z)=(z/σ^2)*e^[-z^2/(2σ^2)],z>=0,f(z)=0,z<0
F(z)=P(Z<=z)=P{(X^2+Y^2)^0.5<=z}
当z<0时，F(z)=0
当z>=0时，F(z)=P{X^2+Y^2<=z^2}
F(z)=P{X^2+Y^2<=z^2}=(2πσ^2)^(-1)∫∫e^[-(x^2+y^2)/(2σ^2)]dxdy,
积分区域是X^2+Y^2<=z^2
积分得概率分布：F(z)=1-e^[-z^2/(2σ^2)]，z>0
求导得概率密度：f(z)=(z/σ^2)*e^[-z^2/(2σ^2)],z>=0,f(z)=0,z<0
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