29.设随机变量 sim N(1,9), sim N(0,16), 且X与Y的相关系数为 (rho )_(xy)=-0.5, =dfrac (1)(3)X+dfrac (1)(2)Y u-|||-求:(1)Cov(X,Y);(4P139,142)-|||-(2)E(Z),D(Z);(4P 126,137)-|||-(3)Cov(X,Z).((4P141)