`***`Lời giải`***`
a)
`x^2-17=0`
`<=>x^2=17`
`<=>`\(\left[ \begin{array}{l}x=\sqrt{17}\\x=-\sqrt{17}\end{array} \right.\)
Vậy `S={\sqrt{17};-\sqrt{17}}`
b)
`x^2-2x\sqrt{17}+17=0`
`<=>(x-\sqrt{17})^2=0`
`<=>x-\sqrt{17}=0`
`<=>x=\sqrt{17}`
Vậy `S={\sqrt{17}}`