3=(3^n)^(1/n)<=(1^n+2^n+3^n)^(1/n)<=(3*3^n)^(1/n)
lim(n→∞)3=3=)<=lim(n→∞)3(3)^(1/n)=3
由挟逼准则知
lim(n→∞)(1^n+2^n+3^n)^(1/n)＝3