`***`Lời giải`***`
a)
`(x-3)^3-(x-3)(x^2+3x+9)+9(x+1)^2=15`
`<=>(x^3-9x^2+27x-27)-(x-3)(x^2+3x+9)+9(x+1)^2=15`
`<=>x^3-9x^2+27x-27-(x^3-27)+9(x^2+2x+1)=15`
`<=>x^3-9x^2+27x-27-x^3+27+9x^2+18x+9=15`
`<=>45x=6`
`<=>x=2/15`
Vậy `S={2/15}`
b)
`x(x-5)(x+5)-(x-2)(x^2+2x+4)=-17`
`<=>x(x^2-25)-(x^3-8)=-17`
`<=>x^3-25x-x^3+8=-17`
`<=>-25x=-17-8`
`<=>-25x=-25`
`<=>x=1`
Vậy `S={1}`