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
