Question

1.2+2.3+3.4+...+38.39+39.40

Answer 1

A    = 1*2 + 2*3 + 3*4 + ... + 38*39 + 39*40

3*A = 1*2*3 + 2*3*3 + 3*4*5 + ... + 38*39*3 + 39*40*3

3*A = 1*2*3 + 2*3*(4 - 1) + 3*4*(5 - 2) + ... + 38*39*(40 - 37) + 39*40*(41 - 38)

3*A = 1*2*3 + 2*3*4 - 1*2*3 + 3*4*5 - 2*3*4 + ... + 38*39*40 - 37*38*39 + 39*40*41 - 38*39*40

3*A = 39*40*41

3*A = 63960

A    = 63960 : 3

A    = 21320

Vậy A = 21320.

Tích đúng cho mk nha ! <3 

Answer 2

Gọi tổng trên là A

A=1.2+2.3+3.4+....+38.39+39.40

3A=1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+...+38.39.(40-37)+39.40.(41-38)

3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+.....+38.39.40-37.38.39+39.40.41-38.39.40

3A=39.40.41

A=(39.40.41)/3=13.40.41=21320