Đáp án:
$\begin{array}{l}
13)9{x^2} - 6x - 3\\
= 9{x^2} - 9x + 3x - 3\\
= 3\left( {x - 1} \right)\left( {3x + 1} \right)\\
14)4{x^2} - 5x + 6\\
= 4{x^2} - 2.2x.\frac{5}{4} + \frac{{25}}{{16}} + \frac{{71}}{{16}}\\
= {\left( {2x - \frac{5}{4}} \right)^2} + \frac{{71}}{{16}}\\
15)2{x^2} - 3x - 2\\
= 2{x^2} - 4x + x - 2\\
= \left( {x - 2} \right)\left( {2x + 1} \right)\\
16)3{x^2} + 10x + 3\\
= 3{x^2} + 9x + x + 3\\
= \left( {x + 3} \right)\left( {3x + 1} \right)\\
17)5{x^2} + 14x - 3\\
= 5{x^2} + 15x - x - 3\\
= \left( {x + 3} \right)\left( {5x - 1} \right)\\
18)6{x^2} + 7x - 3\\
= 6{x^2} + 9x - 2x - 3\\
= \left( {2x + 3} \right)\left( {3x - 1} \right)\\
19){x^2} + xy - 2{y^2}\\
= {x^2} + 2xy - xy - 2{y^2}\\
= \left( {x + 2y} \right)\left( {x - y} \right)\\
20){x^2} - xy - 6{y^2}\\
= {x^2} - 3xy + 2xy - 6{y^2}\\
= \left( {x - 3y} \right)\left( {x + 2y} \right)
\end{array}$
