1/2+1/6+1/12+1/20+1/30+1/42+1/56
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)
=1-1/8=7/8
在原题的基础上 推广 到 n项
即1/2+1/6+………+1/n(n-1)=1-1/n=(n-1)/n