a)Ta có:
\(x\left(x+3\right)=0\Rightarrow\left[{}\begin{matrix}x=0\\x+3=0\Rightarrow x=-3\end{matrix}\right.\)
Vậy \(\left[{}\begin{matrix}x=0\\x=-3\end{matrix}\right.\)
b)Ta có:
\(\left(x-2\right)\left(5-x\right)=0\Rightarrow\left[{}\begin{matrix}x-2=0\\5-x=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=2\\x=5\end{matrix}\right.\)
c)Ta có:
\(\left(x+1\right)\left(x^2+1\right)=0\Rightarrow\left[{}\begin{matrix}x+1=0\\x^2+1=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-1\\\varnothing\end{matrix}\right.\)
Vậy \(x=-1\)