P(Z<=x）=1-P(Z>=x)=1-P(Y>=x)P(X>=x）=2x-x^2从而密度为，2-2x
F(z)=1-(1-Fx(z))(1-Fy(z))=1-(1-z)^2 (0f(z)=dF(z)/dz=2-2z (0其他f(z)=0
