a)
\frac{1}{\sqrt5}=\frac{\sqrt5}{\sqrt5*\sqrt5}=\frac{\sqrt5}{5}
b)
\frac{3+\sqrt5}{\sqrt6}=\frac{(3+\sqrt5)*\sqrt6}{\sqrt6*\sqrt6}=\frac{3\sqrt6+\sqrt{30}}{6}=
c)
\frac{3}{\sqrt5+2}=\frac{3(\sqrt5-2)}{(\sqrt5+2)(\sqrt5-2)}=\frac{3(\sqrt5-2)}{(\sqrt5)^2-2^2}=\frac{3(\sqrt5-2)}{5-4}=3(\sqrt5-2)