b) Rút gọn biểu thức B.

* Hướng dẫn giải

b) $B=\left(\frac{x+3\sqrt{x}-2}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}-\frac{1}{\sqrt{x}+3}\right).\frac{\sqrt{x}-3}{\sqrt{x}+1}=\left(\frac{x+3\sqrt{x}-2-\left(\sqrt{x}-3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\right).\frac{\sqrt{x}-3}{\sqrt{x}+1}$

$=\frac{x+2\sqrt{x}+1}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}.\frac{\sqrt{x}-3}{\sqrt{x}+1}=\frac{{\left(\sqrt{x}+1\right)}^2}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}.\frac{\sqrt{x}-3}{\sqrt{x}+1}=\frac{\sqrt{x}+1}{\sqrt{x}+3}$