$a)\frac{4}{\sqrt{2}}=\frac{4\sqrt{2}}{\sqrt{2}\cdot \sqrt{2}}=\frac{4\sqrt{2}}{2}=2\sqrt{2}$    

$3\sqrt{2}$  

$\frac{5}{2\sqrt{2}}=\frac{5\sqrt{2}}{2\cdot \sqrt{2}\cdot \sqrt{2}}=\frac{5\sqrt{2}}{2\cdot 2}=\frac{5}{4}\sqrt{2}$