a.    12,5 - \(x\) = \(x\) + 12

 \(x\) + 12  + \(x\) = 12,5

  \(x\) x 1 + \(x\) x 1 = 12,5 - 12

  \(x\) x (1 + 1) = 0,5

  \(x\) x 2         = 0,5

  \(x\)               = 0,5 : 2

  \(x\)              = 0,25

b, 8 \(\times\) \(x\) + 3 \(\times\) \(x\) + 2 \(\times\) \(x\) = 20

   \(x\) \(\times\) (8 + 3 + 2)  =20

   \(x\) \(\times\) 13 = 20

  \(x\)          = 20 : 13

  \(x\)          = \(\dfrac{20}{13}\)

b, 8 $\times$ $x$ + 3 $\times$ $x$ + 2 $\times$ $x$ = 20

   $x$ $\times$ (8 + 3 + 2)  =20

   $x$ $\times$ 13 = 20

  $x$          = 20 : 13

  $x$          = 20131320​
 

\(\frac{1}{2}x+\frac{2}{4}x=-\frac{18}{25}.\)

\(\Leftrightarrow\frac{1}{2}x+\frac{1}{2}x=-\frac{18}{25}\)

\(\Leftrightarrow\left(\frac{1}{2}+\frac{1}{2}\right)x=-\frac{18}{25}\)

\(\Leftrightarrow x=-\frac{18}{25}\)

\(5\cdot x+10\cdot9=990\)

\(\Rightarrow5\cdot x+90=990\)

\(\Rightarrow5\cdot x=900\)

\(\Rightarrow x=\dfrac{900}{5}\)

\(\Rightarrow x=180\)

_____________

\(1045:\left[215-\left(3\cdot x-24\right)\right]=5\)

\(\Rightarrow215-\left(3\cdot x-24\right)=1045:5\)

\(\Rightarrow215-\left(3\cdot x-24\right)=209\)

\(\Rightarrow3\cdot x-24=215-209\)

\(\Rightarrow3\cdot x-24=6\)

\(\Rightarrow3\cdot x=30\)

\(\Rightarrow x=10\)

`@` `\text {Ans}`

`\downarrow`

`5.x + 10.9 = 990`

`\Rightarrow 5x + 90 = 990`

`\Rightarrow 5x = 990 - 90`

`\Rightarrow 5x = 900`

`\Rightarrow x = 900 \div 5`

`\Rightarrow x = 180`

Vậy, `x = 180`

\(1045 \div [ 215 - (3 . x - 24 ) ] = 5\)

`\Rightarrow 1045 \div (215 - 3x + 24) = 5`

`\Rightarrow 191 + 3x = 1045 \div 5`

`\Rightarrow 191 + 3x = 209`

`\Rightarrow 3x = 209 - 191`

`\Rightarrow 3x =18`

`\Rightarrow x = 18 \div 3`

`\Rightarrow x = 6`

Vậy, `x = 6.`

5x+10.9=990

=>5x+90=990

=>5x=990-90

=>5x=900

=>x=900:5

=>x=180

______________

1045:[215-(3x-24)]=5

=>215-(3x-24)=1045:5

=>215-(3x-24)=209

=>3x-24=215-209

=>3x-24=6

=>3x=6+24

=>3x=30

=>x=30:3

=>x=10

x*12,5 -x +x *2,75=1050

x*12,5 -x*1+x*2,75=`1050

x*(12,5-1+2,75)=1050

x*14,25=1050

        x=1050:14,25

        x= 73,68421053

a/ => 4x² - 4x + 1 + 4x² + 4x + 1 = 16

=> 8x² = 14

=> x² = 14/8

=> x = \(\frac{\sqrt{7}}{2}\) hoặc x = \(-\frac{\sqrt{7}}{2}\)

b/ => 6x² - (6x² - 11x - 10) = 17

=> 6x² - 6x² + 11x + 10 = 17

=> 11x = 7

=> x = 7/11

c/ => 2x(x + 5) - x² - 5x = 0

=> 2x(x + 5) - x(x + 5) = 0 

=> (x + 5)(2x - x) = 0

=> x(x + 5) = 0 

=> x = 0

hoặc x + 5 = 0 => x = -5

Vậy x = 0 ; x = -5

d/ \(x^2+\frac{1}{x^2}+2x+\frac{2}{x}=-3\)

đề là như vầy hả

smile, giúp mình lần này đi! đi nah! miao~~~~~

\(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+.......+\dfrac{1}{x\cdot\left(x+1\right)}=\dfrac{122}{123}\)

\(\Leftrightarrow1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+......+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{122}{123}\)

\(\Leftrightarrow1-\dfrac{1}{x+1}=\dfrac{122}{123}\)

\(\Leftrightarrow\dfrac{1}{x+1}=\dfrac{1}{123}\)

\(\Leftrightarrow x=122\)

\(x\left(x+y+z\right)=10\)    (1)

\(y\left(y+z+x\right)=25\)  (2)

\(z\left(z+x+y\right)=-10\)  (3)

Lấy  (1) + (2) + (3)  theo vế ta có:

      \(x\left(x+y+z\right)+y\left(y+z+x\right)+z\left(z+x+y\right)=10+25-10\)

\(\Leftrightarrow\)\(\left(x+y+z\right)^2=25\)

\(\Leftrightarrow\)\(x+y+z=\pm\sqrt{25}=\pm5\)

Nếu  \(x+y+z=5\) thì:    \(\hept{\begin{cases}x=2\\y=5\\z=-2\end{cases}}\)

Nếu   \(x+y+z=-5\)thì   \(\hept{\begin{cases}x=-2\\y=-5\\z=2\end{cases}}\)

Vậy...