4.3σ原则-|||-(1)若 sim N(mu ,(sigma )^2), 则对于任何实数 gt 0, (mu -alt Xleqslant -|||-(1+a)=(int )_(a-1)^k+a((x))^k+1(x)dx-|||-(2)正态分布在三个特殊区间内取值的概率:-|||-(mu -sigma lt Xleqslant mu +sigma )approx 0.6827,-|||-(mu -2sigma lt Xleqslant mu +2sigma )approx 0.9545,-|||-(mu -3sigma lt Xleqslant mu +3sigma )approx 0.9973.-|||-(3)通常认为服从于正态分布N(μ,σ^2 )的随机变量X只-|||-取 (mu -3sigma ,mu +3sigma ) 之间的值,并简称之为3σ原则.