（3/4+5/6）×12 =3/4x12+5/6x12=9+10=197/9×5/7＋7/9×2/7 =7/9x(5/7+2/7)=7/9x1=7/915/17×18 =15/17x(17+1)=15/17x17+15/17x1=15+15/17=15又15/173/7-6/13×3/7=3/7x(1-6/13)=3/7x7/13=3/13