X^2 +4x^2/(x+2)^2=12

`(x^2+4x^2)/(x+2)^2=12`  `(đk: xne-2)`

`⇒ x^2+4x^2=12(x+2)^2`

`⇔ 5x^2=12(x^2+4x+4)`

`⇔ 5x^2=12x^2+48x+48`

`⇔ 5x^2-12x^2-48x-48=0`

`⇔ -7x^2-48x-48=0`

`⇔ -7(x^2+48/7x+48/7)=0`

`⇔ x^2+48/7x+48/7=0`

`⇔ (x+24/7)^2-240/49=0`

`⇔ (x+24/7)^2=240/49`

* TH1:

`x+24/7=(√240)/7`

`⇔ x=(√240)/7-24/7`

`⇔ x=(√240-24)/7`

*TH2:

`x+24/7=(- √240)/7`

`⇔ x=(-√240)/7-24/7`

`⇔ x=(-√240-24)/7`

Vậy `S={(±√240-24)/7}`