A = 1/3.5.7 + 1/5.7.9+ ... + 1/2019.2021.2023
2A = 2/3.5.7 + 2/5.7.9+ ... + 2/2019.2021.2023
2A = 1/3-1/5+1/7+1/5-1/7+1/9+....+1/2019-1/2021+1/2023
2A = 1/3 - 1/2023
2A = 2023/6069 - 3/6069
2A = 2023-3/6069
2A = 2020/6069
A = 1010/6069
Vậy A = 1010/6069