设X1,X2,···, _(n)(ngt 2) 为来自总体N(0,1)的简单随机样-|||-本,X为样本均值,记 overline (X)=dfrac (1)(n)sum _(i=1)^n(X)_(i), 记 _(i)=(X)_(i)-overline (X) ,i=1, 2 ,.,-|||-n.求:-|||-(1)Y1的方差D(Y ), i=1, 2,···,n;-|||-(2)Y1与Yn的协方差Cov(Y1,Yn).