S=1.2+2.3+...+39.40

3S=1.2.3+2.3.3+...+39.40.3

3S=1.2.(3-0)+2.3.(4-1)+...+39.40.(41-38)

3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40

3S=(1.2.3+2.3.4+...+39.40.41)-(0.1.2+1.2.3+...+38.39.40)

3S=39.40.41-0.1.2

3S=39.40.41

S=\(\frac{39.40.41}{3}=21320\)

\(S=1\cdot2+2\cdot3+3\cdot4+....+39\cdot40\)
\(\Rightarrow3S=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+....+39\cdot40\cdot\left(41-38\right)\)
\(=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+.....+39\cdot40\cdot41-38\cdot39\cdot40=39\cdot40\cdot41=63960\)

S=21320

chúc bạn học giỏi hơn

mik nha

mik chắc chắn 100%

\(\Rightarrow S=\frac{63960}{3}=21320\)

3S=1.2.3+2.3.(4-1)+...+39.40.(41-38)

=1.2.3+2.3.4+...+39.40.41-1.2.3-...38.39.40(triệt tiêu đi)

=39.40.41

vậy S=\(\frac{39.40.41}{3}\)

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

=1.2.3+2.3.(4-1)+3.4.(5-1)+...+38.39.(40-37)+39.40.(41-58)

=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

=39.40.41=63960

=>A=\(\frac{63960}{3}\)=21320

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 

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

S = 21320

mik nhé

S=1.2+2.3+...+39.40

3S=1.2.(3 - 0)+2.3.(4 - 1)+...+39.40.(41 - 38)

3S=1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 +...+ 39.40.41 - 38.39.40

3S=39.40.41

S=13.40.41

S=21320

euhdewjs