lim(n到无穷）(1+2+3+.+n)/(3n^2) =lim(n到无穷）[(1+n)*n/2]/(3n^2) =lim(n到无穷）1/6[n^2+n]/(n^2) =1/6lim(n到无穷）[1+1/n] =1/6 有疏漏请回复.