1.设x1,x2,x3是取自某总体容量为3的样本,试证下列统计量都是该总体均值μ的无偏估计,-|||-在方差存在时指出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).