18.设x1,x2,···,xn是总体N(μ,σ^2)的简单随机样本.记-|||-overline (X)=dfrac (1)(n)sum _(i=1)^nX , ^2=dfrac (1)(n-1)sum _(i=1)^n(({X)_(i)-overline (X))}^2 , =(overline {X)}^2-dfrac (1)(n)(S)^2-|||-(1)证明T是μ^2的无偏估计量;-|||-(2)当 mu =0 , =1 时,求D(T).