一减二分之三加六分之五减十二分之七加二十分之九= 1 -1/2 + 5/6 - 7/12 + 9 /20= 1 - 1/2 + (1/2 + 1/3) - (1/3 + 1/4) + (1/4 + 1/5)= 1- 1/2 + 1/2 + 1/3 - 1/3 - 1/4 + 1/4 +1/5= 1 +1/5= 6/5
