dô

Lưu ý: ở dạng toán này ta không thể tìm được giá trị chính xác của x, chỉ tìm được giá trị xấp xỉ (gần đúng) của x thôi!\(\dfrac{3-4x}{5-7x}=\dfrac{4x-9}{7x-12}=\dfrac{\left(3-4x\right)+\left(4x-9\right)}{\left(5-7x\right)+\left(7x-12\right)}\)

\(=\dfrac{\left(3-9\right)-\left(4x-4x\right)}{\left(5-12\right)-\left(7x-7x\right)}=\dfrac{-6}{-7}=\dfrac{6}{7}\)(1)

Từ (1) ta có: \(\dfrac{3-4x}{5-7x}=\dfrac{4x-9}{7x-12}=\dfrac{6}{7}\). Từ đây bấm máy Casio.Ta tìm được giá trị xấp xỉ của x là: \(\dfrac{16}{25}\)

a/ 4x(7x - 5) - 7x(4x - 2) = -12

  => 28x² - 20x - 28x² + 14x = -12

  => -6x = -12

  => x = 2

b/ (x + 3)(x - 2) + 3x = 4(x + 3/4) 

  => x² + x - 6 + 3x - 4x - 3 = 0

  => x² - 9 = 0

  => x² = 9

  => x = 3 hoặc x = -3

a) \(4x.\left(7x-5\right)-7x\left(4x-2\right)=-12\)

\(\Rightarrow28x^2-20x-28x^2+14x=-12\)

\(\Rightarrow x\left(14-20\right)=-12\)

\(\Rightarrow-6x=12\)

\(\Rightarrow x=-2\)

b) \(3x.\left(2x-4\right)-6x\left(x+5\right)=x-1\)

\(\Rightarrow3x.\left[\left(2x-4\right)-2.\left(x+5\right)\right]=x-1\)

\(\Rightarrow3x.\left(2x-4-2x-10\right)=x-1\)

\(\Rightarrow-42x=x-1\)

\(\Rightarrow-42x-x=-1\)

\(\Rightarrow-43x=-1\)

\(\Rightarrow x=\dfrac{1}{43}\)

a, \(4x\left(7x-5\right)-7x\left(4x-2\right)=-12\)

\(\Rightarrow28x^2-20x-28x^2+14=-12\)

\(\Rightarrow-20x=-12-14\)

\(\Rightarrow-20x=-26\Rightarrow x=1,3\)

Vậy \(x=1,3\)

b, \(3x\left(2x-4\right)-6x\left(x+5\right)=x-1\)

\(\Rightarrow6x^2-12x-6x^2-30-x=-1\)

\(\Rightarrow-13x=-1+30\)

\(\Rightarrow-13x=29\Rightarrow x=\dfrac{-29}{13}\)

Vậy \(x=\dfrac{-29}{13}\)

Chúc bạn học tốt!!!

4x(7x-5)-7x(4x-2)=-12

\(\Rightarrow\)28\(^{ }\)x^2-20x-28x^2+14x=-12

\(\Rightarrow\)-20x+14x=-12

\(\Rightarrow\)-6x=-12

\(\Rightarrow\)x=2

b, 3x(2x-4)-6x(x+5)=x-1

\(\Rightarrow\)6x^2-12x-6x^2-30x=x-1

\(\Rightarrow\)-42x=x-1

\(\Rightarrow\)-42x-x=-1

\(\Rightarrow\)-43x=-1

\(\Rightarrow\)x=1/43

a) 2x(x - 5) - 2x² = 2x² - 10x - 2x² = -10x = 20 => x = 20 : (-10) = -2

b) 5x(2x - 7) + 2x(8 - 5x) = 10x² - 35x + 16x - 10x² = -19x = 5 => x = \(\frac{5}{-19}=\frac{-5}{19}\)

c) 4x(7x - 5) - 7x(4x - 2) = 28x² - 20x - 28x² + 14x = -6x = -12 => x = -12 : (-6) = 2

MK mới học lớp 7 thôi nhưng mk làm vài câu nha

a) 2x(x-5)-2x²=20

    2x²-100x-2x²=20

    100x=20

     x=20:100

      x=\(\frac{1}{5}\)

a) Ta có : 2x ( x - 5 ) - 2x² = 20

=> 2x ( x - 5 ) - 2x.x = 20

=> 2 x ( x - 5 - x ) = 20

=> 2x . ( -5 ) = 20

=> -10 x = 20

=> x = -2

Chọn mik mik làm tiếp

a)2x(x-5)-2x² = 20

\(VT=-10x\)

\(\Leftrightarrow-10x=20\)

\(\Leftrightarrow x=-2\)

b)5x(2x-7)+2x(8-5x)=5

\(VT=-19x\)

\(\Leftrightarrow-19x=5\)

\(\Leftrightarrow x=-\frac{5}{19}\)

c)4x(7x-5)-7x(4x-2)= -12

\(VT=-6x\)

\(\Leftrightarrow-6x=-12\)

\(\Leftrightarrow x=2\)

a) 4x²  - 2x + 3 - 4x.(x - 5) = 7x - 3

--> 4x² - 2x + 3 - 4x² + 20x = 7x - 3

--> 4x² - 2x - 4x² + 20x - 7x = -3 - 3

--> 11x = -6

--> x = \(\frac{-6}{11}\)

b) -3x.(x - 5) + 5.(x - 1) + 3x² = 4x

--> -3x² + 15x + 5x - 5 + 3x² = 4x

--> -3x²  + 15x + 5x + 3x² - 4x = 5 

--> 16x = 5

--> x = \(\frac{5}{16}\)

c) 7x.(x - 2) - 5.(x - 1) = 21x² - 14x² + 3

--> 7x² - 14x - 5x + 5 = 7x² + 3 

--> 7x²  - 14x - 5x - 7x²  = -5 + 3 

--> -19x = -2 

--> x = \(\frac{2}{19}\)

d) 3.(5x - 1) - x.(x - 2) + x² - 13x = 7

--> 15x - 3 - x² + 2x + x² - 13x = 7

--> 15x - x² + 2x + x² - 13x = 3 + 7

--> 4x = 10

--> x = \(\frac{5}{2}\)

e) \(\frac{1}{5}\)x.(10x - 15) - 2x.(x - 5) = 12

--> 2x² - 3x - 2x² + 10x = 12

--> 7x = 12

--> x = \(\frac{12}{7}\)

~ Học tốt ~

a) 4x² - 2x + 3 - 4x(x - 5) = 7x - 3

=> 4x² - 2x + 3 - 4x² + 20x = 7x - 3

=> 18x + 3 = 7x - 3

=> 18x - 7x = -3 - 3

=> 11x = -6

=>  x = -6/11

b) -3x(x - 5) + 5(x - 1) + 3x² = 4x

=> -3x² + 15x + 5x - 5 + 3x² = 4x

=> 20x - 5 = 4x

=> 20x - 4x = 5

=> 16x = 5

=> x = 5/16

\(c,7x\left(x-2\right)-5\left(x-1\right)=21x^2-14x^2+3\)

\(\Leftrightarrow7x^2-14x-5x+5=7x^2+3\)

\(\Leftrightarrow7x^2-7x^2-19x=3-5\)

\(\Leftrightarrow-19x=-2\)

\(\Leftrightarrow x=\frac{2}{19}\)

a) 4x² - 2x + 3 - 4x.(x - 5) = 7x - 3

<=> 18x + 3 = 7x - 3

<=> 18x = 7x - 3 - 3

<=> 18x = 7x - 6

<=> 18x - 7x = -6

<=> 11x = -6

<=> x = -6/11

=> x = -6/11

b) -3x.(x - 5) + 5.(x - 1) + 3x² = 4x

<=> 20x - 5 = 4x

<=> 20x = 4x + 5

<=> 20x - 4x = 5

<=> 16x = 5

<=> x = 5/16

=> x = 5/16

c) 7x.(x - 2) - 5.(x - 1) = 21x² - 14x² + 3

<=> 7x.(x - 2) - 5.(x - 1) = 7x² + 3

<=> 7x² - 19x + 5 = 7x² + 3

<=> 7x² - 19x = 7x² + 3 - 5

<=> 7x² - 19x = 7x² - 2 

<=> 7x² - 19x - 7x² = -2

<=> -19x = -2

<=> x = 2/19

=> x = 2/19

d) 3.(5x - 1) - x.(x - 2) + x² - 13x = 7

<=> 4x - 3 = 7

<=> 4x = 7 + 3

<=> 4x = 10

<=> x = 10/4

=> x = 5/2

e) 1/5x.(10x - 15) - 2x.(x - 5) = 12

<=> x(2x - 3) - 2x(x - 5) = 12

<=> 7x = 12

<=> x = 12/7

=> x = 12/7