$\displaystyle \begin{array}{{>{\displaystyle}l}} B=\frac{1}{3-\sqrt{x}} +\frac{\sqrt{x}}{3+\sqrt{x}} -\frac{x+9}{ \begin{array}{{>{\displaystyle}l}} x-9\\ \end{array}}\\ ĐK\ x >0;\ x\neq 9\\ B=\frac{3+\sqrt{x} +\sqrt{x}\left( 3-\sqrt{x}\right) +x+9}{x-9}\\ B=\frac{4\sqrt{x} -12}{\left( 3-\sqrt{x}\right)\left(\sqrt{x} +3\right)}\\ B=\frac{4\left(\sqrt{x} -3\right)}{\left( 3-\sqrt{x}\right)\left(\sqrt{x} +3\right)}\\ B=\frac{-4}{\left(\sqrt{x} +3\right)} \end{array}$
