*17/8-4/9+7/8-5/9 =(17/8+7/8)-(4/9+5/9)=24/8 -9/9=3-1=2*13/12-（5/6+1/12)=13/12 -1/12 -5/6=12/12 -5/6=1 -5/6=1/6
