Đáp án:
$\begin{array}{l}
\left( {{x^2} - 2x + 4} \right)\left( {{x^2} + 2x + 4} \right) - 14{x^2}\\
= \left( {{x^2} + 4 - 2x} \right)\left( {{x^2} + 4 + 2x} \right) - 14{x^2}\\
= {\left( {{x^2} + 4} \right)^2} - {\left( {2x} \right)^2} - 14{x^2}\\
= {\left( {{x^2} + 4} \right)^2} - 18{x^2}\\
= {\left( {{x^2} + 4} \right)^2} - {\left( {3\sqrt 2 x} \right)^2}\\
= \left( {{x^2} + 4 - 3\sqrt 2 x} \right)\left( {{x^2} + 4 + 3\sqrt 2 x} \right)
\end{array}$
