2.设X1,X2,X3是取自总体X的样本,试证下列统计量都是总体均值μ的-|||-无偏估计量,并在方差存在时指出哪一个估计最有效?哪一个估计的有效性-|||-最差?-|||-(1) (hat {mu )}_(1)=dfrac (1)(2)(X)_(1)+dfrac (1)(3)(X)_(2)+dfrac (1)(6)(X)_(3),-|||-(2) (hat {mu )}_(2)=dfrac (1)(3)(X)_(1)+dfrac (1)(3)(X)_(2)+dfrac (1)(3)(X)_(3),-|||-(3) (hat {mu )}_(3)=dfrac (1)(6)(X)_(1)+dfrac (1)(6)(X)_(2)+dfrac (2)(3)(X)_(3)