Ta có :

• $\frac{5}{\sqrt{10}}=\frac{5\sqrt{10}}{10}=\frac{\sqrt{10}}{2}$
• $\frac{5}{2\sqrt{5}}=\frac{5\sqrt{5}}{2.5}=\frac{\sqrt{5}}{2}$
•  $\frac{1}{3\sqrt{20}}=\frac{\sqrt{5}}{3\sqrt{20}\sqrt{5}}=\frac{\sqrt{5}}{3.10}=\frac{\sqrt{5}}{30}$
• $\frac{2\sqrt{2}+2}{5\sqrt{2}}=\frac{\sqrt{2}(2+\sqrt{2})}{5\sqrt{2}}=\frac{2+\sqrt{2}}{5}$
• $\frac{y+b\sqrt{y}}{b\sqrt{y}}=\frac{\sqrt{y}(\sqrt{y}+b)}{b\sqrt{y}}=\frac{\sqrt{y}+b}{b}$