ta có : B = 1.2.3 + 2.3.4 + 3.4.5 + ...... + 2016.2017.2018
4B = 1.2.3.4 - 1.2.3.4 + 2.3.4.5 - 2.3.4.5 + ...... + 2016.2017.2018.2019
4B = 2016.2017.2018.2019
vậy B = 2016.2017.2018.2019/4
Ta có : B = 1.2.3 + 2.3.4 + ...... + 2016.2017.2018
=> 4B = 1.2.3.4 - 1.2.3.4 + 2.3.4.5 - 2.3.4.5 + ...... + 2016.2017.2018.2019
=> 4B = 2016.2017.2018.2019
=> B = 2016.2017.2018.2019/4