a)
\frac{7-2\sqrt5}{7+2\sqrt5}=\frac{(7-2\sqrt5)(7-2\sqrt5)}{(7+2\sqrt5)(7-2\sqrt5)}=\frac{(7-2\sqrt5)^2}{49-4*5}=
\frac{49-28\sqrt5+4*5}{49-20}=\frac{69-28\sqrt5}{29}
b)
\frac{\sqrt6-1}{2\sqrt5}=\frac{(\sqrt6-1)*\sqrt5}{2\sqrt5*\sqrt5}=\frac{\sqrt{30}-\sqrt5}{2*5}=\frac{\sqrt{30}-\sqrt5}{10}