Đáp án:
$\rm 2018.2019.505.2021$
Giải thích các bước giải:
Đặt $\rm S= 1.2.3+2.3.4+3.4.5+.....+2018.2019.2020$
$\rm 4S=1.2.3.4+2.3.4.4+....+2018.2019.2020.4$
$\rm 4S=1.2.3.4+2.3.4.(5-1)+....+2018.2019.2020.(2021-2017)$
$\rm 4S=1.2.3.4+2.3.4.5-1.2.3.4+...+2018.2019.2020.2021-2017.2018.2019.2020$
$\rm 4S=2018.2019.2020.2021$
$\rm S=\dfrac{2018.2019.2020.2021}{4}=2018.2019.505.2021$
