7/9+6/7+2/9-3/7=(7/9+2/9)+(6/7-3/7)=9/9+3/7=1+3/7=1又3/7
7/9+6/7+2/9-3/7=7/9+2/9+6/7-3/7=1+3/7=10/7=1又3/7