Đặt A = 1.2 + 2.3 + 3.4 + ... + 2019.2020
=> 3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 2019.2020.3
= 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + .... + 2019.2020.(2021 - 2018)
= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + .... + 2019.2020.2021 - 2018.2019.2020
= 2019.2020.2021
=> A = 2019.2020.2021 : 3 = 2 747 468 660