S²=1/n[(x1-x拔) ²+（x2-x拔）²+……+(xn-x) ²]
=1/n[(x1²+x2²+……xn²)+n*(xn²)-2x拔(x1+x2+x3+……xn）]
=1/n[(x1²+x2²+……xn²)+n*(x拔²)-2x拔²*n]
=1/n[(x1²+x2²+……xn²)-n*x拔²]