Đáp án đúng: A Giải chi tiết:Rút gọn biểu thức \(P = \frac{{\sqrt x }}{{\sqrt x + 3}} - \frac{3}{{\sqrt x - 3}} + \frac{{6\sqrt x }}{{x - 9}}\) (với \(x \ge 0,x \ne 9\) ). \(\begin{array}{l}P = \frac{{\sqrt x }}{{\sqrt x + 3}} - \frac{3}{{\sqrt x - 3}} + \frac{{6\sqrt x }}{{x - 9}}\\ = \frac{{\sqrt x .\left( {\sqrt x - 3} \right)}}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}} - \frac{{3.\left( {\sqrt x + 3} \right)}}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}} + \frac{{6\sqrt x }}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}}\\ = \frac{{x - 3\sqrt x - 3\sqrt x - 9 + 6\sqrt x }}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}}\\ = \frac{{x - 9}}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}} = \frac{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}}{{\left( {\sqrt x + 3} \right).\left( {\sqrt x - 3} \right)}} = 1.\end{array}\) Vậy \(P = 1.\) Chọn A.