1/2=1/1-1/21/6=1/2-1/31/12=1/3-1/4...一直拆分到1/72=1/8-1/9相加，相互抵消，剩下1/1-1/9=8/9
1/6+1/12+1/20+1/30+1/42+1/56=8/9
