5/12-3/7-4/7+7/12=（5+7）/12 -（3+4）/7=1-1=0
(5/12-3/7)-(4/7-7/12)=5/12-3/7-4/7+7/12=5/12+7/12-3/7-4/7=(5/12+7/12)-(3/7+4/7)=12/12-7/7=1-1=0
