设X1,X2,···, _(n)(ngt 2) 为独立同分布的随机变量,且均服-|||-从N(0,1).记 overline (X)=dfrac (1)(n)sum _(i=1)^nX ,_(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);-|||-(3) ((Y)_(1)+(Y)_(n)leqslant 0) -