乘以3分之4减去3乘以5分之8加上5乘以7分之12减去7乘以9分之16
=4/3 - (4/3 - 4/5) + (6/5 -6/7) - (8/7 - 8/9)
=4/5 + 6/5 - (6/7 + 8/7) + 8/9
=2 - 2 + 8/9
=8/9