Đáp án:
`x^8+2x^6-12x^4-13x^2+42=0`
`<=>x^8-2x^6+4x^6-8x^4-4x^4+8x^2-21x^2+42=0`
`<=>x^6(x^2-2)+4x^4(x^2-2)-4x^2(x^2-2)-21(x^2-2)=0`
`<=>(x^2-2)(x^6+4x^4-4x^2-21)=0`
`**x^2-2=0`
`<=>x=+-sqrt2`
`**x^6+4x^4-4x^2-21=0`
`<=>x^6+3x^4+x^4+3x^2-7x^2-21=0`
`<=>x^4(x^2+3)+x^2(x^2+3)-7(x^2+3)=0`
`<=>(x^2+3)(x^4+x^2-7)=0`
`<=>x^4+x^2-7=0`
Đặt `a=x^2(a>=0)`
`pt<=>a^2+a-7=0`
`<=>a=(-1+\sqrt{29})/2(do \ a>=0)`
`<=>x=+-\sqrt{(sqrt{29}-1)/2}`.
Vậy `S={sqrt2,-sqrt2,-\sqrt{(sqrt{29}-1)/2},\sqrt{(sqrt{29}-1)/2}}`
