P(-3<=ξ<=-1）=0.4即φ[（-1+1）/σ]-φ[（-3+1）/σ]=0.4而φ[（-1+1）/σ]=φ（0）=0.5则φ[（-3+1）/σ]=0.1即φ[-2/σ]=0.1而正态分布函数为偶函数，所以φ[-2/σ]=φ[2/σ]=0.1所以P（ξ>=1）=1-φ[(1+1）/σ]=1-0.1=0.9
