4.设总体 sim N(mu ,(sigma )^2), x1,x2,···,xn为样本,证明 overline (x)=dfrac (1)(n)sum _(i=1)^n(x)_(i) ^2=dfrac (1)(n-1)sum _(i=1)^n(({x)_(i)-overline (x))}^2 分别为-|||-mu ,(sigma )^2的UMVUE..