`1) x^2+4x=0`
`<=>x(x+4)=0`
`<=>` \(\left[ \begin{array}{l}x=0\\x=-4\end{array} \right.\)
Vậy `S={-4;0}`
`2) x^2+4x+4=0`
`<=> (x+2)^2=0`
`<=> x+2=0`
`<=> x=-2`
Vậy `S={2}`
`3) x^2-8=0`
`<=> x^2=8`
`<=> x=\sqrt{8}`
`<=> x=+-2\sqrt{2}`
Vậy `S={-2\sqrt{2};2\sqrt{2}}`
`4) x^2+3=0`
`<=> x^2=-3` (vô lí)
`<=> x=\emptyset`
Vậy `S =\emptyset`
`5) 9x^2-6x+1=0`
`<=> (3x-1)^2=0`
`<=> 3x-1=0`
`<=> 3x=1`
`<=> x=1/3`
Vậy `S={1/3}`
