1/4-9/20+11/30-13/42+15/56
=1/4-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)
=1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8
=1/8=1/4-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)=1/4-1/4+1/5-1/5+1/6-1/6+1/7-1/7+1/8=1/8因该是这个吧恩，对的