Cho biểu thức A= ( căn x / (căn x+3)+(2 căn x)/( căn x-3)-(3x+9)/(x-9)

* Hướng dẫn giải

$\begin{array}{l}a)A=\frac{\sqrt{x}}{\sqrt{x}+3}+\frac{2\sqrt{x}}{\sqrt{x}-3}-\frac{3x+9}{x-9}\\ =\frac{\sqrt{x}}{\sqrt{x}+3}+\frac{2\sqrt{x}}{\sqrt{x}-3}-\frac{3x+9}{\left(\sqrt{x}-3\right).\left(\sqrt{x}+3\right)}=\frac{\sqrt{x}\left(\sqrt{x}-3\right)+2\sqrt{x}\left(\sqrt{x}+3\right)-3x-9}{\left(\sqrt{x}-3\right).\left(\sqrt{x}+3\right)}\\ =\frac{x-3\sqrt{x}+2x+6\sqrt{x}-3x-9}{\left(\sqrt{x}-3\right).\left(\sqrt{x}+3\right)}=\frac{3\sqrt{x}-9}{\left(\sqrt{x}-3\right).\left(\sqrt{x}+3\right)}=\frac{3\left(\sqrt{x}-3\right)}{\left(\sqrt{x}-3\right).\left(\sqrt{x}+3\right)}=\frac{3}{\sqrt{x}+3}\\ \\ \end{array}$