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

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

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

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

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

\(\Rightarrow S=21320\)

sao chưa ai trl hết v:)))

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!^-^

\(S=1.2+2.3+3.4+...+38.39+39.40\)

\(3\text{S}=1.2.3+2.3.\left(4-1\right)+...+38.39.\left(40-37\right)+39.40.\left(41-38\right)\)

\(3\text{S}=1.2.3+2.3.4-1.2.3+...+38.39.40-37.38.39+39.40.41-38.39.40\)

\(3\text{S}=39.40.41\)

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

      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

21320