2又1/5-4/7-3/7=2又1/5-(4/7+3/7)=2又1/5-1=1又1/51又5/6-2/9-7/9=1又5/6-(2/9+7/9)=1又5/6-1=5/62又4/9-3/11-4/9=2又4/9-4/9-3/11=2-3/11=1又8/11