4.设随机向量(X,Y)服从二元正态分布N(-|||-((mu )_(1),(mu )_(2);({sigma )_(1)}^2,({sigma )_(2)}^2;rho ), 其中 (mu )_(1)=1, (mu )_(2)=2,,-|||-({sigma )_(1)}^2=2 , ({O)_(2)}^2=8, ρ=0.2, 则有 X-2Y 亦服从正态分布,为N( __ ,__