tính giá trị biểu thức A = 3/2- 5/6+ 7/12-9/20+11/30-13/42+15/56-17/72+19/90

`A = 3/2 - 5/6 + 7/12 - 9/20 + 11/30  - 13/42 + 15/56 - 17/72 + 19/90`

`A = 3/(1.2) - 5/(2.3) + 7/(3.4) - 9/(4.5) + 11/(5.6) - 13/(6.7) + 15/(7.8) - 17/(8.9) + 19/(9.10)`

`A = 3 - 3/2 - 5/2 - 5/3 + 7/3 - 7/4 + 9/4 - 9/5 + 11/5 - 11/6 - 13/6 - 13/7 + 15/7 - 15/8 - 17/8 - 17/9 + 19/9 - 19/10`

`A = 3 - (3/2 + 5/2) + (5/3 + 7/3) - (7/9 + 9/4) - (9/5 + 11/5) - (11/6 + 13/6) + (13/7 + 15/7) + (15/8 + 17/8) - (17/9 + 19/9) - 19/10`

`A = 3 - 4 + 4 - 4 + 4 - 4  + 4  - 4  + 4  - 4  + 4  - 19/10`

`A = 3 - 19/10`

`A = 30/10 - 19/10`

`A = 11/10`