Rút gọn biểu thức \(A.\) A.\(A = \frac{3}{{\sqrt x - 3}}\) B.\(A = \frac{3}{{\sqrt x + 3}}\) C.\(A = \frac{{\sqrt x - 3}}{{\sqrt x + 3}}\) D.\(A = \frac{{\sqrt x + 3}}{{\sqrt x - 3}}\)
Đáp án đúng: B Giải chi tiết:Điều kiện: \(x \ge 0,\,\,\,x \ne 9.\) \(\begin{array}{l}A = \frac{{\sqrt x }}{{\sqrt x + 3}} + \frac{{2\sqrt x }}{{\sqrt x - 3}} - \frac{{3x + 9}}{{x - 9}}\\\,\,\,\, = \frac{{\sqrt x }}{{\sqrt x + 3}} + \frac{{2\sqrt x }}{{\sqrt x - 3}} - \frac{{3x + 9}}{{\left( {\sqrt x - 3} \right)\left( {\sqrt x + 3} \right)}}\\\,\,\,\, = \frac{{\sqrt x \left( {\sqrt x - 3} \right) + 2\sqrt x \left( {\sqrt x + 3} \right) - 3x - 9}}{{\left( {\sqrt x + 3} \right)\left( {\sqrt x - 3} \right)}}\\\,\,\,\, = \frac{{x - 3\sqrt x + 2x + 6\sqrt x - 3x - 9}}{{\left( {\sqrt x + 3} \right)\left( {\sqrt x - 3} \right)}}\\\,\,\, = \frac{{3\sqrt x - 9}}{{\left( {\sqrt x + 3} \right)\left( {\sqrt x - 3} \right)}} = \frac{{3\left( {\sqrt x - 3} \right)}}{{\left( {\sqrt x + 3} \right)\left( {\sqrt x - 3} \right)}} = \frac{3}{{\sqrt x + 3}}.\end{array}\) Chọn B.