(1)设随机变量X1,X 2,X3相互独立,且有 _(1)approx b(4,dfrac (1)(2)) _(2)approx -|||-(6,dfrac (1)(3)), _(3)approx b(6,dfrac (1)(3)), 求 {X)_(1)=2,(X)_(2)=2,(X)_(3)=5} , E(X1X2X3), ((X)_(1)--|||-_(2)), ((X)_(1)-2(X)_(2)),-|||-(2)设X,Y是随机变量,且有 E(X)=3 (Y)=1 (x)=4 (x)=9,-|||-令 =5x-y+15, 分别在下列3种情况下求E(Z)和D(Z),-|||-(i)X,Y相互独立,(ii)X,Y不相关,(ii)X与Y的相关系数为0.25.