Question

Tính giá trị biẻu thức:

A= 1.2 + 2.3 +3.4 +...+ 18.19

Answer 1

=> 3A = 1.2. (3-0) +2.3.(4-1) + ...+18.19.(20-17)

=1.2.3-0.1.2+2.3.4-1.2.3+...+18.19.20-17.18.19

=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...+(17.18.19-17.18.19)+18.19.20-0.1.2

=0+0+0+...+0+18.19.20

=18.19.20

=> A = 6.19.20= 114* 20= 2280

 

Answer 2

`A= 1.2 + 2.3 +3.4 +...+ 18.19`

`3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) +....+ 18.19.(20-17)`

`3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ....+ 18.19.20 - 17.18.19 `

`3A = 18.19.20`

`A = 6.19.20`

`A = 2280`

Answer 3

Đặt `A=1.2+2.3+...+18.19`

`3A=1.2(3-0)+2.3(4-1)+...+18.19(20-17)`

`3A=1.2.3-0.1.2+2.3.4-1.2.3+...+18.19.20-17.18.19`

`3A=18.19.20=6840`

\(\Rightarrow\) `A=6840/3`

\(\Rightarrow\) `A=2280`

`@Nae`