x3 + 27 + (x + 3)(x - 9) = 0
(x + 3)(x2 - 3x + 9) + (x + 3)(x - 9) = 0
(x + 3)(x2 - 3x + 9 + x - 9) = 0
(x + 3)(x2 - 2x) = 0
x(x + 3)(x - 2) = 0
\(\left[\begin{array}{nghiempt}x=0\\x+3=0\\x-2=0\end{array}\right.\)
\(\left[\begin{array}{nghiempt}x=0\\x=-3\\x=2\end{array}\right.\)
\(=\left(x+3\right)\left(x^2+9-3x\right)+\left(x+3\right)\left(x-9\right)\)
\(=\left(x+3\right)\left(x^2-2x\right)=x\left(x-2\right)\left(x+3\right)=0\)
\(\Rightarrow\left[\begin{array}{nghiempt}x=0\\x=2\\x=-3\end{array}\right.\)