Question

tính tổng:1.2+2.3+.....+2019.2020

ai nhanh tick liền,giúp mik vs

Answer 1

A = 1.2 + 2.3 + ... + 2019.2020

3A = 1.2.3 + 2.3.(4-1) + ... + 2019.2020.(2021-2018)

3A = 1.2.3 + 2.3.4 - 1.2.3 + ... + 2019.2020.2021 - 2018.2019.2020

3A = 2019.2020.2021

A = 2747468660

Answer 2

Đặt S=1.2+2.3+..........+2019.2020

3S=1.2.3+2.3.3+....+2019.2020.3

3S=1.2.3+2.3(4-1)+....+2019.2020(2021-2018)

3S=1.2.3+2.3.4-1.2.3+....+2019.2020.2021-2018.2019.2020

3S=(1.2.3-1.2.3)+(2.3.4-2.3.4)+....+(2018.2019.2020-2018.2019.2020)+2019.2020.2021

3S=2019.2020.2021

S=(2019.2020.2021):3

Answer 3

CON NGU BÀI DỄ THẾ KO BIẾT LÀM