11设(X1,X2,···,,nn )为来自总体 approx U(theta -0.5,theta +0.5) 的-|||-样本 (-infty lt theta lt 00).-|||-(1)求未知参数θ的极大似然估计;-|||-(2)证明UND和 dfrac (1)(2)((X)_((1))+(X)_((n))) 都是θ的无偏估计量,问-|||-哪一个较有效?