5/6 - 7/12 + 9/20 - 11/30 + 13/42 - 15/56 + 17/72= (1/2 + 1/3) - (1/3+1/4) + (1/4+1/5) - (1/5 + 1/6) + (1/6 + 1/7) - (1/7 +1/8) + (1/8 +1/9)= 1/2 + 1/9 （中间的全部抵消了）= 11/18答案：18分之11...