Câu 1: $y = \cos \left( {x - \dfrac{\pi }{3}} \right)\cos \left( {x + \dfrac{\pi }{4}} \right) + \cos \left( {x + \dfrac{\pi }{6}} \right)\cos \left( {x + \dfrac{{3\pi }}{4}} \right)$.

Câu 2: $y = {\cos ^2}x + {\cos ^2}\left( {\dfrac{{2\pi }}{3} + x} \right) + {\cos ^2}\left( {\dfrac{{2\pi }}{3} - x} \right)$.