Câu 66:
$A = \sin x\cos y + \sin y\cos x = \sin \left( {x + y} \right) = \sin {30^o} = \dfrac{1}{2}$. D
Câu 67:
$A = \sin x\sin y + \cos x\cos y = \cos \left( {y - x} \right) = \cos {180^o} = - 1$. C
Câu 68:
$\begin{array}{l} y = \sin \alpha \cos \alpha \cos 2\alpha \cos 4\alpha = \dfrac{1}{2}.2\sin \alpha \cos \alpha \cos 2\alpha \cos 4\alpha = \dfrac{1}{2}\sin 2\alpha \cos 2\alpha .\cos 4\alpha \\ y = \dfrac{1}{4}\sin 4\alpha \cos 4\alpha = \dfrac{1}{8}\sin 8\alpha \to B \end{array}$
