Question

[Hoàng Nghĩa Phạm]

Trả lời: ........

Answer 1

[Phạm Công Thành]

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

<=>3S= 1.2.3 +2.3.3+ 3.4.3 +...+ 38.39.3 + 39.40.3

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

<=>3S= 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

<=> 3S=39.40.41

<=> S= \(\dfrac{39.40.41}{3}\)

<=>S=21320