Đáp án:
`A=1010/2021`
Giải thích các bước giải:
`A=1/(1×3)+1/(3×5)+1/(5×7)+...+1/(2019×2021`
`2A=2/(1xx3)+1/(3xx5)+1/(5xx7)+...+1/(2019xx2021)`
`2A=1/1-1/3+1/3-1/5+1/5-1/7+...+1/2019-1/2021`
`2A=1/1-1/2021`
`2A=2021/2021-1/2021`
`2A=2020/2021`
`A=2020/2021:2`
`A=2020/2021 . 1/2`
`A=2020/4042`
`A=1010/2021`