设X1,X2是来自任意总体UND的一个容量为2的-|||-样本, =dfrac (2)(3)(X)_(1)+dfrac (1)(3)(X)_(2) =dfrac (1)(4)(X)_(1)+dfrac (3)(4)(X)_(2),-|||-=dfrac (2)(5)(X)_(1)+dfrac (2)(5)(X)_(2), =dfrac (1)(2)((X)_(1)+(X)_(2)).-|||-(1)确定上面4个E (X)的估计量中,那些是E(X-|||-)的无偏估计量-|||-(2)在无偏估计量中找出最有效的估计量,并说-|||-明原因。