Đáp án đúng: B
Giải chi tiết:\(\begin{array}{l}4)\,\,S = \left( {\frac{{\sqrt x + 3}}{{\sqrt x - 2}} + \frac{{\sqrt x + 2}}{{3 - \sqrt x }} + \frac{{\sqrt x + 2}}{{x - 5\sqrt x + 6}}} \right):\left( {1 - \frac{{\sqrt x }}{{\sqrt x + 1}}} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {DK:\,\,x \ge 0,\,\,x \ne 4,\,\,x \ne 9} \right)\\\,\,\,\,\,\,\,\,\,\,\, = \left( {\frac{{\sqrt x + 3}}{{\sqrt x - 2}} - \frac{{\sqrt x + 2}}{{\sqrt x - 3}} + \frac{{\sqrt x + 2}}{{(\sqrt x - 3).(\sqrt x - 2)}}} \right):\left( {\frac{{\sqrt x + 1 - \sqrt x }}{{\sqrt x + 1}}} \right)\\\,\,\,\,\,\,\,\,\,\, = \frac{{(\sqrt x + 3)(\sqrt x - 3) - (\sqrt x + 2)(\sqrt x - 2) + \sqrt x + 2}}{{(\sqrt x - 3).(\sqrt x - 2)}}:\frac{1}{{\sqrt x + 1}}\\\,\,\,\,\,\,\,\,\,\, = \frac{{x - 9 - (x - 4) + \sqrt x + 2}}{{(\sqrt x - 3).(\sqrt x - 2)}}:\frac{1}{{\sqrt x + 1}}\\\,\,\,\,\,\,\,\,\, = \frac{{x - 9 - x + 4 + \sqrt x + 2}}{{(\sqrt x - 3).(\sqrt x - 2)}}:\frac{1}{{\sqrt x + 1}}\\\,\,\,\,\,\,\,\, = \frac{{\sqrt x - 3}}{{(\sqrt x - 3).(\sqrt x - 2)}}.(\sqrt x + 1)\\\,\,\,\,\,\,\,\, = \frac{{\sqrt x + 1}}{{\sqrt x - 2}}.\end{array}\)