由已知，f(x)=1/2, (-1 f(x)=0, 其他FY(y)=P{Y<=y}=P{2-X<=y}=P{X>=2-y}=1-P{X<2-y}fY(y)=F'Y(y)=[1-FX(2-y)]'=f(2-y)[1-(2-y)]'=1/2 所以fY(y)=1/2,1<=y<3 =0,其他
