x/2+x/6+x/12+x/20+……+x/(n-1)n=(1/2+1/6+1/12+1/20+…………+1/(n-1)n)x=(1/1*2+1/2*3+1/3*4+1/4*5+…………+1/（n-1）n)x=(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+…………+1/（n-1）-1/n)x=(1-1/n)x>n-1x*(n-1)/n>n-1x...