Đáp án:
$\begin{array}{l}
P = \frac{{x - 13}}{{\sqrt {x - 9} - 2}}\left( {x \ne 13;x \ge 9} \right)\\
= \frac{{\left( {x - 13} \right)\left( {\sqrt {x - 9} + 2} \right)}}{{\left( {\sqrt {x - 9} - 2} \right)\left( {\sqrt {x - 9} + 2} \right)}}\\
= \frac{{\left( {x - 13} \right)\left( {\sqrt {x - 9} + 2} \right)}}{{x - 9 - {2^2}}}\\
= \frac{{\left( {x - 13} \right)\left( {\sqrt {x - 9} + 2} \right)}}{{x - 13}}\\
= \sqrt {x - 9} + 2
\end{array}$