Trục căn thức ở mẫu: 

- Hướng dẫn giải

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