Tìm x biết: x^3 + 27 + (x + 3)(x – 9) = 0.

Trả lời

x³ + 27 + (x + 3)(x - 9) = 0

⇔ x³ + 3³ + (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

$\iff \left[\begin{array}{l}x=0\\ x+3=0\\ x-2=0\end{array}\right.\iff \left[\begin{array}{l}x=0\\ x=-3\\ x=2\end{array}\right.$

Vậy  $x\in \left\{0;-3;2\right\}$.