`x/3-x^2/4=0`
`->(4x-3x^2)/12=0`
`->4x-3x^2=0`
`->x(4-3x)=0`
`->`\(\left[ \begin{array}{l}x=0\\4-3x=0\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=0\\x=\frac{3}{4}\end{array} \right.\)
Vậy `S={0;3/4}`
`(x^2+1)(2x+1)+2x=1`
`->2x^3+x^2+2x+1+2x=1`
`->2x^3+x^2+4x=0`
`->x(2x^2+x+4)=0`
Vì `2x^2+x+4>0`
`->x=0`
Vậy `S={0}`