1/2=1-1/2 1/2=1-1/25/6=1-1/6 1/6=1/2-1/311/12=1-1/12 1/12=1/3-1/419/20=1-1/20 1/20=1/4-1/5 2/1+6/5+12/11+20/19+30/29+42/41+56/55 =(1-1/2)+(1-1/6)……+(1-1/56)=(1+1+1+……+1)+(1/2+1/6+1/12+……+1/56)=7+(1-1/8)=7又7/8