7/9+2/3-(1/3+4/9)=7/9+2/3-1/3-4/9=(7/9-4/9)+(2/3-1/3)=3/9+1/3=1/3+1/3=2/3回答完毕~！请笑纳！
(7/9-4/9)+(2/3-1/3)=3/9+1/3=2/3
