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f(x)=e^-x f(y)=e^-y即F(x)=1-e^-x F(y)=1-e^-yF(z)=P{Z<=z}=1-P{Z>z}=1-P{X>z}P{Y>z}=1-[1-F(z)][1-F(z)]=1-e^-2z , z>0故f(z)=2e^-2z , z>0 f(z)=0 , z<=0