Question

Tính giá trị của biểu thức :

     A = 1*2+2*3+3*4+...+2019*2020

Answer 1

A=1.2+2.3+3.4+.............+2019.2020

3A=1.2.3+2.3.3+3.4.3+........................+2019.2020.3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+..............+2019.2020.(2021-2018)

3A=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+.............-2018.2019.2020+2019.2020.2021

3A=2019.2020.2021

A=\(\frac{2019.2020.2021}{3}\)

A=2747468660

Vậy A=2747468660

Chúc bn học tốt

Answer 2

\(A=1.2+2.3+3.4+.......+2019.2020\)

\(\Rightarrow3A=1.2.3+2.3.3+3.4.3+......+2019.2020.3\)

\(=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+.........+2019.2020.\left(2021-2018\right)\)

\(=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+.......+2019.2020.2021-2018.2019.2020\)

\(=2019.2020.2021\)

\(\Rightarrow A=\frac{3A}{3}=\frac{2019.2020.2021}{3}=2747468660\)

Vậy \(A=2747468660\)

Answer 3

A=1.2+2.3+3.4+.............+2019.2020 3A=1.2.3+2.3.3+3.4.3+........................+2019.2020.3 3A=1.2.3+2.3.(4-1)+3.4.(5-2)+..............+2019.2020.(2021-2018) 3A=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+.............-2018.2019.2020+2019.2020.2021 3A=2019.2020.2021 A= 3 2019.2020.2021 A=2747468660 Vậy A=2747468660 Chúc bn học tốt