$\frac{a}{\sqrt{11}}=\frac{a\ast <!--\; \ast \; -->\sqrt{11}}{\sqrt{11}\ast <!--\; \ast \; -->\sqrt{11}}=\frac{a\sqrt{11}}{(\sqrt{11}{)}^2}=\frac{a\sqrt{11}}{11}$

$\frac{18}{\sqrt{6}}=\frac{18\ast <!--\; \ast \; -->\sqrt{6}}{\sqrt{6}\ast <!--\; \ast \; -->\sqrt{6}}=\frac{18\sqrt{6}}{(\sqrt{6}{)}^2}=\frac{18\sqrt{6}}{6}=3\sqrt{6}$

$\frac{5}{\sqrt{10}}=\frac{5\ast <!--\; \ast \; -->\sqrt{10}}{\sqrt{10}\ast <!--\; \ast \; -->\sqrt{10}}=\frac{5\sqrt{10}}{(\sqrt{10}{)}^2}=\frac{5\sqrt{10}}{10}=\frac{\sqrt{10}}{2}$

$\frac{13}{\sqrt{26}}=\frac{13\ast <!--\; \ast \; -->\sqrt{26}}{\sqrt{26}\ast <!--\; \ast \; -->\sqrt{26}}=\frac{13\sqrt{26}}{(\sqrt{26}{)}^2}=\frac{13\sqrt{26}}{26}=\frac{\sqrt{26}}{2}$

$\frac{30}{\sqrt{15}}=\frac{30\ast <!--\; \ast \; -->\sqrt{15}}{\sqrt{15}\ast <!--\; \ast \; -->\sqrt{15}}=\frac{30\sqrt{15}}{(\sqrt{15}{)}^2}=\frac{30\sqrt{15}}{15}=2\sqrt{15}$

$\frac{2}{3\sqrt{x}}=\frac{2\ast <!--\; \ast \; -->\sqrt{x}}{3\sqrt{x}\ast <!--\; \ast \; -->\sqrt{x}}=\frac{2\sqrt{x}}{3(\sqrt{x}{)}^2}=\frac{2\sqrt{x}}{3x}$