你好:1/4+5/9-4/9+3/4 =(1/4+3/4)+(5/9-4/9)=1+1/9=1又1/9
1/4+4/4+5/9-4/9=1+1/9=10/9
1/4+5/9-4/9+3/4 =(1/4+3/4)+(5/9-4/9)=1+1/9=1又1/9