Вычисли

а) \( \cos 60\degree - \sqrt{6}\cos 30\degree \sin 45\degree + \ctg 30\degree \tg 150\degree - \tg 45\degree = \) [ ];

б) \(\sin \dfrac{\pi}{6} + \sqrt{2} \cos \dfrac{3\pi}{4}-\sqrt{3} \ctg \cfrac{\pi}{3}= \) [ ].