x/2+x/6+x/12+x/20+...+x/(n-1)n>n-1x×[1/2+1/6+1/12+1/20+……+1/(n-1)n]＞n-1x×[1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+……+1/(n-1)-1/n]＞n-1(1-1/n)x＞n-1(n-1)x/n＞n-1x＞n
