Vì `a = 1/2021` nên ta có `:`
`P = | a - 1/2020 | + | a - 1/2022 |`
`P = | 1/2021 - 1/2020 | + | 1/2021 - 1/2022 |`
`P = | 1/2020 - 1/2021 | + | 1/2021 - 1/2022 |`
`P = 1/2020 - 1/2021 + 1/2021 - 1/2022`
`P = 1/2020 - 1/2022`
`P = 1/2042220`
Vậy `, P = 1/2042220 .`