=2-|||-4.设X1,X2,···,x3是来自总体 approx N(1,4) 的简单随机样本, overline (X)=dfrac (1)(n)sum _(i=1)^4(X)_(i) =sum _(i=1)^n(({x)_(i)-overline (x))}^2, 则统计景-|||-12(X5-X)^2 服从 __ -