\(C=2x^3+x^2+x-1=0\\ \Rightarrow x^3+x^3+x^2+x-1=0\\ \Rightarrow x^3+\left(x^3+x^2\right)-\left(x+1\right)=0\\ \Rightarrow x^3+x^2\left(x+1\right)-\left(x+1\right)=0\\ \Rightarrow x^3+\left(x+1\right)\left(x^2-1\right)=0\\ \Rightarrow\left\{{}\begin{matrix}x^3=0\\\left(x+1\right)\left(x^2-1\right)=0\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=0\\\left[{}\begin{matrix}x+1=0\\x^2-1=0\end{matrix}\right.\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=0\\\left[{}\begin{matrix}x=-1\\x=\sqrt{1}\end{matrix}\right.\end{matrix}\right.\)
Vậy đa thức trên có nghiệm là \(\left\{{}\begin{matrix}x=0\\\left[{}\begin{matrix}x=-1\\x=\sqrt{1}\end{matrix}\right.\end{matrix}\right.\)