因为1-1/2=1/21/2-1/3=1/61/3-1/4=1/12………………所以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）+（1/4-1/5）+（1/5-1/6）+(1/6-1/7)+(1/7-1/8)=1-1/8=7/8
