1/20=1/4-1/5
1/30=1/5-1/6
以此类推
1/12+1/20+1/30+1/42+1/56
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1/3-1/8
=5/24