Giải hộ mình trong tối nay với

`a, A = 1/1.3 + 1/3.5 + 1/5.7 + ... + 1/2019.2021`

`=> 2A = 2(1/1.3 + 1/3.5 + 1/5.7 + ... + 1/2019.2021)`

`=> 2A = 2/1.3 + 2/3.5 + 2/5.7 + ... + 2/2019.2021`

`=> 2A = 1 - 1/3 + 1/3 - 15 + 1/5 - 1/7 + ... + 1/2019 - 1/2021`

`=> 2A = 1 - 1/2021`

`=> 2A = 2020/2021`

`=> A = 2020/2021 . 1/2`

`=> A = 1010/2021`

  Vậy `A = 1010/2021`

`b, B = 3/1.3 + 3/3.5 + 3/5.7 +...+ 3/499.501`

`=> 2/3B = 2/3 (3/1.3 + 3/3.5 + 3/5.7 +...+ 3/499.501)`

`=> 2/3B = 2/1.3 + 2/3.5 + 2/5.7 + ... + 2/499.501`

`=> 2/3B = 1 - 1/3 + 1/3 - 15 + 1/5 - 1/7 + ... + 1/499 - 1/501`

`=> 2/3B = 1 - 1/501`

`=> 2/3B = 500/501`

`=> B = 500/501 . 3/2`

`=> B = 250/167`

  Vậy `B = 250/167`