Đáp án+Giải thích các bước giải:
`x^4+6x^3+11x^2+6x+1=0`
`<=>x^4+3x^3+x^2+3x^3+9x^2+3x+x^2+3x+1=0`
`<=>x^2(x^2+3x+1)+3x(x^2+3x+1)+x^2+3x+1=0`
`<=>(x^2+3x+1)(x^2+3x+1)=0`
`<=>(x^2+3x+1)^2=0`
`<=>x^2+3x+1=0`
`<=>x^2+2.x. 3/2+9/4=5/4`
`<=>(x+3/2)^2=5/4`
`+)x+3/2=(\sqrt{5})/2`
`<=>x=(\sqrt{5}-3)/2`
`+)x+3/2=(-\sqrt{5})/2`
`<=>x=(-\sqrt{5}-3)/2`
Vậy `S={(\sqrt{5}-3)/2,(-\sqrt{5}-3)/2}`