a, A = 2012 . 2018
=> A = ( 2014 - 2 ) . 2018
=> A = 2014.2018 - 2.2018
b, B = 2014 . 2016
=> B = 2014 . ( 2018 - 2 )
=> B = 2014 . 2018 - 2014 .2
Vì 2.2018 > 2 .2014
=> A > B
b, A = 2019 . 2021
=> A = ( 2020 - 1 ) . 2021
=> A = 2020.2021 - 2021
b, B = 20202
=> B = 2020 . 2020
=> B = ( 2021 - 1 ) . 2020
=> B = 2021.2020 - 2020
Vì 2020.2021-2021 < 2021.2020 - 2020
=> A < B
Câu a, mik làm nhầm tí nhé :
Sửa Vì 2014 . 2018 - 2.2018 < 2014.2018 - 2.2014
=> A < B