Rút gọn biểu thức \[A = \sqrt x \left( {\frac{1}{{\sqrt x + 3}} - \frac{1}{{3 - \sqrt x }}} \right)\] (x ≥ 0, x ≠ 9).

Ta có \[A = \sqrt x \left( {\frac{1}{{\sqrt x + 3}} - \frac{1}{{3 - \sqrt x }}} \right)\]

\[ = \sqrt x \cdot \frac{{3 - \sqrt x - \sqrt x - 3}}{{\left( {\sqrt x + 3} \right)\left( {3 - \sqrt x } \right)}}\]

\[ = \sqrt x .\frac{{ - 2\sqrt x }}{{{3^2} - {{\left( {\sqrt x } \right)}^2}}} = \frac{{ - 2x}}{{9 - x}}.\]