Đáp án + Giải thích các bước giải:

$\textbf{9}\bigg)$

$\begin{array}{l} &x(x + y) - 5x - 5y
\\ &= x(x + y) - 5(x + y)
\\ &= (x - 5)(x + y)\end{array}$ 

$\textbf{10}\bigg)$

$\begin{array}{l} &x^2 - 2x + 1 - y^2
\\ &= (x - 1)^2 - y^2
\\ &= (x - 1 + y)(x - 1 - y)\end{array}$ 

$\textbf{11}\bigg)$

$\begin{array}{l} &x^2 + 6x + 9 - y^2
\\ &= (x + 3)^2 - y^2
\\ &= (x + 3 + y)(x + 3 - y)\end{array}$ 

$\textbf{12}\bigg)$

$\begin{array}{l} &x^2 - 6x - 4y^2 + 9
\\ &= x^2 -6x + 9 - 4y^2
\\ &= (x - 3)^2 - (2y)^2
\\ &= (x - 3 + 2y)(x - 3 - 2y)\end{array}$