5/7-5/9×5/7=5/7×(1-5/9)=5/7×4/9=20/631-5/7×21/25=1-3/5=2/51/2+5/4×4/5=1/2+1=1又1/21/6×5-2/3=5/6-2/3=1/67/8×7+7/8=7/8×(7+1)=7/8×8=7