Tính

a)$\frac{\sqrt{15}-\sqrt{6}}{\sqrt{35}-\sqrt{14}}$

b)$\frac{2\sqrt{15}-2\sqrt{10}+\sqrt{6}-3}{2\sqrt{5}-2\sqrt{10}-\sqrt{3}+\sqrt{6}}$
c)$\frac{\sqrt{10}+\sqrt{15}}{\sqrt{8}+\sqrt{12}}$​