11/3-7/12+9/20-11/30+13/42-15/56+17/72

一点也不复杂.7/12=1/3+1/4 9/20= 以此类推 所以原式=11/3-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+(1/8+1/9) =11/3-1/3+1/9 =10/3+1/9 =31/9