2.9 验证式(2.63):-|||-forall omega t((e)_(i))=[ 1-dfrac (1)(n)-dfrac ({({x)_(i)-overline (x))}^2}({L)_(w)}] (sigma )^2-|||-2.10用2.9题证明 (hat {sigma )}^2=dfrac (1)(n-2)sum _(i=1)^n(({y)_(i)-overline (y)i)}^2 是σ^2的无偏估计。