Question

 s = 1x2+2x3+........+38x39+39x40 

hỏi s bằng bao nhiêu

Answer 1

Ta có 

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

=> S = \(\frac{38.39.40}{3}\)

=> S = 21320

Answer 2

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

3S = 1x2x3 + 2x3x3 + ... + 38x39x3 + 39x40x3

3S = 1x2x3 + 2x3x(4-1 ) + ... + 38x39x(40-37) + 39x40x(41-38)

3S = 1x2x3 + 2x3x4 - 1x2x3 + ... + 38x39x40 - 37x38x39 + 39x40x41 - 38x39x40

3S = 39x40x41

S = 39x40x41 : 3

S = 21320

Answer 3

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