Ta có :
A = 2020 . 2020 - ( 2020 + 2 ) . 2018
A = 2020 . 2020 - 2018 . 2020 - 2018 . 2 = 2020 . ( 2020 - 2018 ) - 2018 .2
A = 2 . ( 2020 - 2018 ) = 2 . 2 = 4
B = -3/4 . -8/9 . -15/16....-399/400
=( -1 ) ^19 .1.3/2.2 . 2.4/3.3 .3.5/4.4 .... 19.21/20.20
=( -1). (1.2.3....19).(3.4.5...21)/(2.3.4....20).(2.3.4....20)
=( -1 ) . 1.21/20.2 = -21/40
XIN HAY NHẤT Ah