Đáp án:
(1 . 2 + 2 . 3 + ... + 2017 . 2018 + 2018 . 2019) - (1² + 2² + ... + 2017² + 2018²)
= (1 . 2 + 2 . 3 + ... + 2017 . 2018 + 2018 . 2019) - (1 . 1 + 2 . 2 + ... + 2017 . 2017 + 2018 . 2018)
= [1 . (1 + 1) + 2 . (2 + 1) + ... + 2017 . (2017 + 1) + 2018 . (2018 + 1)] - (1 . 1 + 2 . 2 + ... + 2017 . 2017 + 2018 . 2018)
= (1 . 1 + 1 + 2 . 2 + 2 + ... + 2017 . 2017 + 2017 + 2018 . 2018 + 2018) - (1 . 1 + 2 . 2 + ... + 2017 . 2017 + 2018 . 2018)
= 1 + 2 + ... + 2017 + 2018
= 2 037 171.
Chúc học tốt!!!
