3/5-5/9+2/5-4/9=(3/5+2/5)-(5/9+4/9)=5/5-9/9=1-1=0
3/5-5/9+2/5-4/9=3/5+2/5-(5/9+4/9)=1-1=0
解:原式=27/45-25/45+18/45-20/45=0/45=0