Đáp án:
$\begin{array}{l}
{x^9} - {x^7} + {x^6} - {x^5} - {x^4} + {x^3} - {x^2} + 1\\
= {x^7}\left( {{x^2} - 1} \right) + \left( {{x^6} - {x^4}} \right) - \left( {{x^5} - {x^3}} \right) - \left( {{x^2} - 1} \right)\\
= {x^7}\left( {{x^2} - 1} \right) + {x^4}\left( {{x^2} - 1} \right) - {x^3}\left( {{x^2} - 1} \right) - \left( {{x^2} - 1} \right)\\
= \left( {{x^2} - 1} \right)\left( {{x^7} + {x^4} - {x^3} - 1} \right)\\
= \left( {{x^2} - 1} \right)\left( {{x^3} + 1} \right)\left( {{x^4} - 1} \right)\\
= \left( {{x^2} - 1} \right)\left( {{x^3} + 1} \right)\left( {{x^2} - 1} \right)\left( {{x^2} + 1} \right)\\
= {\left( {{x^2} - 1} \right)^2}\left( {{x^2} + 1} \right)\left( {{x^3} + 1} \right)
\end{array}$
