简单3/2
3/2-5/6+7/12-9/20+11/30-13/42
(1+1/2)-(1/2+1/3)+(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7) =1+1/2-1/2-1/3+1/3+1/4-1/4-1/5+-1/5-+/6-1/6-1/7 =1-1/7 =6/7