`A= 2021/2 + 2021/6 + 2021/12 +...+ 2021/9900`
`A= 2021( 1/2 + 1/6+ 1/12 +... + 1/9900)`
`A= 2021( 1/1.2 + 1/2.3 + 1/3.4 +...+ 1/99.100)`
`A= 2021( 1/1 - 1/2 + 1/2 - 1/3 +...+ 1/99- 1/100)`
`A= 2021( 1 - 1/100)`
`A = 2021 . 99/100`
`A=200079/100`
Vậy `A=200079/100`