Answer 1

\(x^3+4x=0\)

\(\Leftrightarrow x\cdot\left(x^2+4\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x^2+4=0\left(L\right)\end{matrix}\right.\)

1 nghiệm.

Answer 2

\(x^3+4x=0\)

\(\Leftrightarrow x(x^2+4)=0\)

\(\Leftrightarrow \left[\begin{array}{l}x=0\\x^2+4=0\end{array}\right.\)

\(\Leftrightarrow \left[\begin{array}{l}x=0\ (tm)\\ x^2=-4\ (ktm)\end{array}\right.\)

Vậy \(S=\{0\}\)

\(\to\) Đa thức chỉ có \(1\) nghiệm