Question

 

tính S= 1x2+2x3+3x4+.......+38x39+39x40

 

Answer 1

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

3.S= 3.(1.2+2.3+3.4+.....+38.39+39.40)

3S = 1.2.3+2.3.3+3.3.4+......+3.38.39+3.39.40

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

3S = 1.2.3+2.3.4-2.3.1+3.4.5-3.4.2+......+38.39.40-38.39.37+39.40.41-39.40.38

3S = (1.2.3-2.3.1)+(2.3.4-3.4.2)+.....+(38.39.40-39.40.38)+39.40.41

3S = 39.40.41

S = (39.40.41) :3

S = (39:3).40.41

S = 13.40.41=21320

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Answer 2

S=21320

bài này mk làm rồi ,tại lươi không muốn giải ~???@!*^_^

Answer 3

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

3S = 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

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

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

S = 39 x 40 x 41 : 3

S = 21320

~ Hok tốt ~

Answer 4

S=21320

~Hok tốt~

Answer 5

#)Giải :

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

=> 3S = 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 

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

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

=> S = 39 x 40 x 41 : 3

=> S = 21320  

                  #~Will~be~Pens~#
 

Answer 6

trả lời

S=21320 

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