Đặt \(x^2+2x+3=a\)
\(a^2-9a+18=0\)
\(\Leftrightarrow\left(a-6\right)\left(a-3\right)=0\)
\(\Leftrightarrow a=6;a=3\)
\(\left(x^2+2x+3\right)^2-9\left(x^2+2x+3\right)+18=0\)
\(\Leftrightarrow\left(x^2+2x+3\right)^2-3\left(x^2+2x+3\right)-6\left(x^2+2x+3\right)+18=0\)
\(\Leftrightarrow\left(x^2+2x+3\right)\left(x^2+2x+3-4\right)-6\left(x^2+2x+3-3\right)=0\)
\(\Leftrightarrow\left(x^2+2x\right)\left(x^2+2x+3-6\right)=0\)
\(\Leftrightarrow x\left(x+2\right)\left(x+3\right)\left(x-1\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x=0\\x+2=0\end{cases}}\) hoặc \(\orbr{\begin{cases}x+3=0\\x-1=0\end{cases}}\)
\(\Leftrightarrow x\in\left\{0,-2,-3,1\right\}\)
