kết quả:

S = 1.843279e61

mik không chắc lắm về kết quả

vì mik nhác vào lấy máy tính nên dùng máy tính của điện thoại

nhanh đi 1 k ngay nếu đúng có cần lời giải

mk thấy đề sai hay sao ý

Ta có :

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

\(\Rightarrow3S=1.2.3+2.3.3+....+39.40.3\)

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

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

\(\Rightarrow3S=39.40.41\)

\(\Rightarrow S=\frac{38.39.40}{3}=21320\)

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

x=21320

S = 21320

Ta có:
S = 1x2 + 2x3 + 3x4 + ...+ 38x39 + 39x40
S x 3 = 1x2x3 + 2x3x3 + 3x4x3 +… + 38x39x3 + 39x40x3
S x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)
S x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + ... + 38x39x40 - 37x38x39 + 39x40x41 - 38x39x40.
S x 3 = 39x40x41
S = 39x40x41:3= 21320

ta có: S=2+6+12+1482+1560

 S=3062

chúc bạn học giỏi,nhớ k cho mình

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

2 + 2 x 4 + 38 x 39 + 39 x 40=

2 + 8 +38 x 39 +39 x40=

2 + 8 + 1482 + 39 x 40=

2 + 8+ 1482 + 1560=

10 +1482 +1560 =

1492 +1560 = 3052

Ta có:
S = 1x2 + 2x3 + 3x4 + ...+ 38x39 + 39x40
S x 3 = 1x2x3 + 2x3x3 + 3x4x3 +… + 38x39x3 + 39x40x3
S x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)
S x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + ... + 38x39x40 - 37x38x39 + 39x40x41 - 38x39x40.
S x 3 = 39x40x41
S = 39x40x41:3= 21320

Đặt A = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40

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

           = 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

           = 39.40.41 = 63960

=> A = 63960 : 3 = 21320

1 x 40 x (2 x 2) + (3x2) + (4x2) +...+ ( 39 x 2)

1 x 40 x (2 + 3 + 4 + .... + 39) x 2

40    x 779 x 2

31160 x 2

62320

k nha

 từ đề suy ra 7x-7+3x-6=-3

suy ra 10x-13+3=0

suy ra 10x-10=0

suy ra 10x=10

suy ra x=1

\(7\left(x-1\right)+3.\left(x-2\right)=-3\)

\(\Rightarrow7x-7+3x-6=-3\)

\(\Rightarrow\left(7x+3x\right)+\left(-7-6\right)=-3\)

\(\Rightarrow10x-13=-3\)

\(\Rightarrow10x=10\)

\(\Rightarrow x=1\)

Vậy x=1.

_Học tốt_

7. ( x - 1)+ 3 . ( x - 2 ) = -3

7x - 7 + 3x - 6= -3

7x +3x = -3 +7+6

10x =10

x = 10:10

x= 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

\(3S=1.2.3+2.3.3+...+39.40.3\)

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

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

\(3S=30.40.41\)

\(S=10.40.41\)

<span class="label label-info"><i class="fa fa-exclamation-triangle fa-2"></i> Quản lý</span>

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

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

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

=> 3S = 39 x 40 x 41

=> S = 39 x 40 x 41 : 3

=> S = 21320