`a) A = 2019. 2021`
`= (2020 - 1). 2021`
`= 2020. 2021 - 2021`
`= 2020. 2021 - 2020 - 1`
`B = 2020^2`
`= 2020. 2020`
`= (2021 - 1). 2020`
`= 2020. 2021 - 2020`
Vì `2020. 2021 - 2020 - 1 < 2020. 2021 - 2020`
`=> A < B`
Vậy `A < B`
`b) M = 2008. 2012`
`N = 2009. 2011`
`= (2008 + 1). 2011`
`= 2011. 2008 + 2011`
`= 2011. 2008 + 2008. 1 + 3`
`= 2008. (2011 + 1) + 3`
`= 2008. 2012 + 3`
Vì `2008. 2012 < 2008. 2012 + 3`
`=> M < N`
Vậy `M < N`