`2-25x^2=0`
⇔`(\sqrt{2}-5x)(\sqrt{2}+5x)=0`
⇔\(\left[ \begin{array}{l}\sqrt{2}-5x=0\\\sqrt{2}+5x=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=\dfrac{\sqrt{2}}{5}\\x=-\dfrac{\sqrt{2}}{5}\end{array} \right.\)
Vậy `S={\sqrt{2}/(5),-(\sqrt{2})/(5)}`