Ta có:
$\begin{array}{l}
3{x^2} - 22xy - 4x + 8y + 7{y^2} + 1\\
= 3{x^2} - 21xy - 3x - xy + 7{y^2} + y - x + 7y + 1\\
= 3x\left( {x - 7y - 1} \right) - y\left( {x - 7y - 1} \right) - \left( {x - 7y - 1} \right)\\
= \left( {3x - y - 1} \right)\left( {x - 7y - 1} \right)
\end{array}$
