Question

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

nhanh nha cần có lời giải và đúng sẽ cho 1 tick

Answer 1

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

3S = 1x2x3 + 2x3x3 + 3x4x3 + ... + 38x39x3 + 39x40x3

3S = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)

3S = 1x2x3 + 2x3x4-1x2x3 + 3x4x5-2x3x4 + ... + 38x39x40-37x38x39 + 39x40x41-38x39x40

S = 39x40x41 : 3

S = 21320

Answer 2

21320 nhé bạn

Answer 3

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

3S = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 38.39.(40-37) + 39.40.(41-38)

3S = 1.2.3 - 0.1.2 + 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 = (1.2.3 + 2.3.4 + 3.4.5 + ... + 38.39.40 + 39.40.41) - (0.1.2 + 1.2.3 + 2.3.4 + ... + 37.38.39 + 38.39.40)

3S = 39.40.41 - 0.1.2

3S = 39.40.41

S = 13.40.41

S = 21320

Answer 4

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

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

=> 3S là: 

3S = 1x2x3 + 2x3x3 + 3x4x3 + ... + 38x39x3 + 39x40x3

3S = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)

3S = 1x2x3 + 2x3x4-1x2x3 + 3x4x5-2x3x4 + ... + 38x39x40-37x38x39 + 39x40x41-38x39x40

S = 39x40x41 : 3

S = 21320

Answer 5

S = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40
3S = 1.2.﴾3‐0﴿ + 2.3.﴾4‐1﴿ + 3.4.﴾5‐2﴿ + ... + 38.39.﴾40‐37﴿ + 39.40.﴾41‐38﴿
3S = 1.2.3 ‐ 0.1.2 + 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 = ﴾1.2.3 + 2.3.4 + 3.4.5 + ... + 38.39.40 + 39.40.41﴿ ‐ ﴾0.1.2 + 1.2.3 + 2.3.4 + ... + 37.38.39 + 38.39.40﴿
3S = 39.40.41 ‐ 0.1.2
3S = 39.40.41
S = 13.40.41
S = 21320

Học tốt nha hí hí

Answer 6

S = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40
3S = 1.2.﴾3‐0﴿ + 2.3.﴾4‐1﴿ + 3.4.﴾5‐2﴿ + ... + 38.39.﴾40‐37﴿ + 39.40.﴾41‐38﴿
3S = 1.2.3 ‐ 0.1.2 + 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 = ﴾1.2.3 + 2.3.4 + 3.4.5 + ... + 38.39.40 + 39.40.41﴿ ‐ ﴾0.1.2 + 1.2.3 + 2.3.4 + ... + 37.38.39 + 38.39.40﴿
3S = 39.40.41 ‐ 0.1.2
3S = 39.40.41
S = 13.40.41
S = 21320

Answer 7

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

ta có :

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

3S = 1x2x3 + 2x3x3 + 3x4x3 + ... + 38x39x3 + 39x40x3

3S = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)

3S = 1x2x3 + 2x3x4-1x2x3 + 3x4x5-2x3x4 + ... + 38x39x40-37x38x39 + 39x40x41-38x39x40

S = 39x40x41 : 3

S = 21320

Answer 8

S=21320

Answer 9

S=21320

chắc chắn lun !!!!

Answer 10

mấy người cóp bài nhau à ?

Answer 11

21320

Answer 12

 Sao lại nhân 3 nhở ._.