图
5/6 ×1/7+1/6×1/7=(5/6+1/6)×1/7=1×1/7=1/73/4 ×1/5 ×4=3/20 ×4=3/587 ×3/86=(86+1)×3/86=3+3/86=3又3/861/11 ×2/9+2/11 ×4/9 +6/11×2/9= 1/11 ×2/9+4/11 ×2/9 +6/11×2/9=(1/11+4/11+6/11)×2/9=11/11 ×2/9=2/9