tìm x a) 4x^2-9 b) x^3-9x^2 +27x-35=0 c) x^2-6x-7 d) x^2+4x+3

`a) 4x^2-9`

`= (2x)^2 - 3^2 `

`=  (2x+3). (2x-3)`

`b) x^3-9x^2 +27x-35=0`

` ⇔x^3 - 3. x^2 . 3 + 3. x. 3^2 - 3^3 -8 =0`

`⇔(x-3)^3 = 8`

`⇔(x-3)^3 = 2^3`

`⇔x-3 = 2`

`⇔x= 5`

Vậy `x=5` 

`c) x^2-6x-7`

`=x^2 -7x + x -7`

`=x(x-7) + (x-7) .1`

`=(x-7)(x+1)`

`d) x^2+4x+3`

`=x^2 + x+3x+3`

`=x(x+1) + 3(x+1)`

`=(x+1)(x+3)`