Question

Tính: s=1x2+2x3+3x4+...+38x39+39x40
Trả lời:  s=..........

Answer 1

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

=>S=21320

Answer 2

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

\(\Rightarrow3S=3\left(1.2+2.3+3.4+...+39.40\right)\)

\(\Rightarrow3S=3.1.2+3.2.3+3.3.4+...+3.39.40\)

\(\Rightarrow3S=\left(3-0\right).1.2+\left(4-1\right).2.3+\left(5-2\right).3.4+...+\left(41-38\right).39.40\)

\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+39.40.41-38.39.40\)

\(\Rightarrow3S=39.40.41\)

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

Answer 3

21320

Answer 4

21320

bạn nhé

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LOL

Answer 5

 S = 21320

CHẮC LUÔN MK THI VIOLIMPIC RÙI

Answer 6

S = 21320 nhà bạn 

tk nha 

xin đó

Answer 7

dfsfgf

ghf

Answer 8

21320 nhà bạn

Answer 9

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

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

S x 3 = 1 x 2 x ( 3 - 0 ) + 2 x 3 x ( 4 - 1 ) + 3 x 4 x ( 5 - 2 ) +........ + 38 x 39 x ( 40 - 37 ) + 39 x 40 x ( 41 - 38 )

S x 3 = 1 x 2 x 3 - 1 x 2 x 0 + 2 x 3 x 4 - 2 x 3 x 1 + 3 x 4 x 5 - 2 x 3 x 4 + ..... + 38 x 39 x 40 - 38 x 39 x 37 + 39 x 40 x 41 - 39 x 40 x 38

S x 3 = ( 1 x 2 x 3 - 2 x 3 x 1 ) + ( 2 x 3 x 4 - 2 x 3 x 4 ) + ( 3 x 4 x 5 - 3 x 4 x 5 ) + ........ + ( 38 x 39 x 40 - 39 x 40 x 38 ) + 39 x 40 x 41

S x 3 = 39 x 40 x 41

S x 3 = 63960

     S  = 63960 : 3

     S  = 21320

Vậy : S = 1 x 2 + 2 x 3 + 3 x 4 + ..... + 38 x 39 + 39 x 40 = 21320

Answer 10

đáp án là:

S = 21230

chúc bạn hok tốt !!!