Rút gọn:
a) \(\left(\dfrac{\sqrt{2}+5}{\sqrt{2}-5}-\dfrac{\sqrt{2}-5}{\sqrt{2}+5}\right)\) : \(\dfrac{\sqrt{2}}{23}\)
b) \(\dfrac{\sqrt{2\left(\sqrt{2}-\sqrt{7}\right)^2}}{\sqrt{56}-4}\)
c) \(\dfrac{\sqrt{3\left(3-11\right)^2}}{\sqrt{6}\left(3-11\right)^2}\)