\(\Leftrightarrow x\left(x+5\right)\left(x+1\right)\left(x+4\right)-12=0\)
\(\Leftrightarrow\left(x^2+5x\right)\left(x^2+5x+4\right)-12=0\)
Đặt \(x^2+5x=t\)
\(\Rightarrow t\left(t+4\right)-12=0\)
\(\Leftrightarrow t^2+4t-12=0\Rightarrow\left[{}\begin{matrix}t=2\\t=-6\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x^2+5x-2=0\\x^2+5x+6=0\end{matrix}\right.\) (bấm casio)