giúp e gấp ạ e cảm ơn

`19,` `(2-x)(x+1)=(x-2)(3x+5)`

`<=> (2-x)(x+1)-(x-2)(3x+5) = 0`

`<=> (2-x)(x+1)+(2-x)(3x+5)=0`

`<=>(2-x)(x+1+3x+5)=0`

`<=> (2-x)(4x+6)=0`

`<=>` $\left[\begin{matrix} 2-x=0\\4x+6=0\end{matrix}\right.$

`<=>` $\left[\begin{matrix} x=2\\x=-\dfrac{3}{2}\end{matrix}\right.$

Vậy `S = { 2 ; -3/2 }`

`21,` `(6x-7)(3x+4)=(7-6x)(x-1)`

`<=> (6x-7)(3x+4)-(7-6x)(x-1)=0`

`<=> (6x-7)(3x+4)+(6x-7)(x-1)=0`

`<=> (6x-7)(3x+4+x-1)=0`

`<=> (6x-7)(4x+3)=0`

`<=>` $\left[\begin{matrix}6x-7=0\\4x+3=0\end{matrix}\right.$

`<=>`  $\left[\begin{matrix} x=\dfrac{7}{6}\\x=-\dfrac{3}{4}\end{matrix}\right.$

Vậy `S = { 7/6 ; -3/4 }`

`23,` `(2x^2+1)(4x-3)=(2x^2+1)(x-12)`

`<=> (2x^2+1)(4x-3)-(2x^2+1)(x-12)=0`

`<=> (2x^2+1)[4x-3-(x-12)]=0`

`<=> (2x^2+1)(4x-3-x+12)=0`

`<=> (2x^2+1)(3x+9)=0`

Vì `2x^2 + 1 > 0 ∀ x`

`=> 3x + 9 = 0`

`<=> x = -3`

Vậy `S = { -3}`