7.已知X1,X2为来自总体X的样本, (mu )_(1)=(X)_(1)+(X)_(2) , (mu )_(2)=0.2(X)_(1)+0.8(X)_(2) ,-|||-.(mu )_(3)=0.4(x)_(1)+0.6(x)_(2) 为总体均值的估计量,则其中无偏估计量是 __ ,最有-|||-效的估计量是