`a) x^3-0,25x=0`
`<=>x(x^2-0,25)=0`
`<=>`$\left[\begin{matrix} x=0\\ x^2-0,25=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=0\\ x^2=0,25\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=0\\ x^2=(0,5)^2\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=0\\ x=\pm0,5\end{matrix}\right.$
Vậy `S={0; +-0,5}`
`b)` Giống câu `a`
