60-60除以5+6 用脱式计算
12/7 -5/6 -2/7 -1/6=12/7 -2/7 -(5/6 +1/6)=10/7 -1=3/7(3/4)/(3/4 -1/2)=1/[1-(1/2)x(4/3)]=1/(1- 2/3)=1/(1/3)=3
