设随机变量x1,x2,x 3,x4相互独立,且有 ((x)_(i))=i,-|||-((x)_(i))=5-i;i=1,2,3,4, 设-|||-=2(x)_(1)-(x)_(2)+3(x)_(3)-dfrac (1)(2)(x)_(4) 求E(Y),D(Y)