Answer = 21320 

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

=12130

nha bn!

ý lộn,21320

Ta có :

\(S=1.2+2.3+....+39.40\)

\(\Leftrightarrow3S=1.2.\left(3-0\right)+2.3.\left(4-1\right)+....+39.40.\left(41-38\right)\)

\(\Leftrightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+.....+39.40.41-38.39.40\)

\(\Leftrightarrow3S=39.40.41\)

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

Ta có :

S=1X2+2X3+.........+38X39+39X40

=> 3S = 1X2X3+2X3X3+........+38X39X3+39X40X3

=>3S = 1X2X(3-0)+2X3X(4-1)+.........+38X39X(40-37)+39X40X(41-38)

=> 3S = 1X2X3-0X1X2+2X3X4-1X2X3+...+38X39X40-37X38X39 +39X40X41-38X39X40

=>3S = 39X40X41=63960

=>S = 63960:3

S = 21320

21320

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

=> 3S = 63960

=> S = 21320

Vậy S là 21320

Ta có

S=1.2+2.3+....+39.40S=1.2+2.3+....+39.40

⇒3S=1.2(3−0)+2.3(4−1)+....+39.40(41−38)⇒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=1.2.3−0.1.2+2.3.4−1.2.3+....+39.40.41−38.39.40

⇒ S = 39.40.41/3 ⇒ S = 39.40.41/3

=> S = 21320

Vậy S = 21320

s la21320

21320

=21320

K mình nha.Thank you very much

Good luck

21320

S = 1 x 2 + 2 x 3 + 3 x 4 + ... + 38 x 39 + 39 x 40

Ta có

S=1.2+2.3+....+39.40S=1.2+2.3+....+39.40

⇒3S=1.2(3−0)+2.3(4−1)+....+39.40(41−38)⇒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=1.2.3−0.1.2+2.3.4−1.2.3+....+39.40.41−38.39.40

⇒S=39.40.413⇒S=39.40.41/3

=> S = 21320

Vậy S=21320

\(S=1.2+2.3+...+38.39\)

\(\Rightarrow3S=1.2\left(3-0\right)+2.3\left(4-1\right)+...+39.40\left(41-38\right)\)

\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40\)

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

\(\Rightarrow S=21320\)

kết quả thôi nhé

150558

k cho mình nhé!

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=39.40.41

S=13.40.41

S=21320

ai nhanh mình kich

      S = 1*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

=>3S = 63960

=> S = 21320

S =21320 bạn ạ

k nha

s=21320 bạn ạ

k mik nha

thank you very much!^-^