Câu 1
2)
ĐKXĐ: $\left\{ \begin{array}{l} \sqrt x - 3 \ne 0\\ \sqrt x + 3 > 0\\ \sqrt x \ne 0\\ x \ge 0 \end{array} \right. \Rightarrow \left\{ \begin{array}{l} x \ne 9;0\\ x > 0 \end{array} \right. \Rightarrow x > 0,x \ne 9$
b)
$\begin{array}{l} A = \left( {\dfrac{1}{{\sqrt x + 3}} + \dfrac{1}{{\sqrt x - 3}}} \right).\dfrac{{\sqrt x + 3}}{{\sqrt x }}\\ A = \left( {\dfrac{{\sqrt x - 3 + \sqrt x + 3}}{{\left( {\sqrt x - 3} \right)\left( {\sqrt x + 3} \right)}}} \right).\dfrac{{\sqrt x + 3}}{{\sqrt x }}\\ A = \dfrac{{2\sqrt x }}{{\left( {\sqrt x - 3} \right)\left( {\sqrt x + 3} \right)}}.\dfrac{{\sqrt x + 3}}{{\sqrt x }}\\ A = \dfrac{2}{{\sqrt x - 3}}\\ \Rightarrow \dfrac{2}{{\sqrt x - 3}} = 1 \Leftrightarrow \sqrt x - 3 = 2\\ \Leftrightarrow \sqrt x = 5\\ \Leftrightarrow x = 25 \end{array}$
