`A = 2021/2022`
`= (2022 - 1)/2022`
`= 2022/2022 - 1/2022`
`= 1 - 1/2022`
`B = (2020 xx 2021)/(2020 xx 2021 + 1)`
`= (2020 xx 2021 + 1)/(2020 xx 2021 + 1) - 1/(2020 xx 2021 + 1)`
`= 1 - 1/(2020 xx 2021 + 1)`
Vì `2022 < 2020 xx 2021 + 1`
`=> 1/2022 > 1/(2020 xx 2021 + 1)`
`=> - 1/2022 < - 1/(2020 xx 2021 + 1)`
`=> 1 - 1/2022 < 1 - (2020 xx 2021 + 1)`
`=> A < B`