7.设X1,X2,···,,,,xn为总体N(0,1)的样本,样本均值为UND,-|||-记 _(i)=(x)_(i)-overline (X),-|||-求: 1)D(Y1);2)cov(Y1,Yn ):-|||-3)若 (({r)_(1)+(r)_(n))}^2 是σ^2的无偏估计量,确定c-|||-4)求 ((Y)_(1)+(Y)_(n)lt 0)