15.设总体 sim N(mu ,(sigma )^2), X1,X2,X3是X一个样本,试验证-|||-(hat {mu )}_(1)=dfrac (1)(5)(X)_(1)+dfrac (3)(10)(X)_(2)+dfrac (1)(2)(X)_(3) , (mu )_(2)=dfrac (1)(3)(X)_(1)+dfrac (1)(4)(X)_(2)+dfrac (5)(12)(X)_(3),-|||-(hat {mu )}_(3)=dfrac (1)(3)(X)_(1)+dfrac (1)(6)(X)_(2)+dfrac (1)(2)(X)_(3)-|||-都是μ的无偏估计量,并分析哪一个最好.