Trả lời bởi giáo viên
Đáp án A: $\left. \begin{array}{l}\sin (\pi - \alpha ) = \sin \alpha \\\sin (\pi + \alpha ) = - \sin \alpha \end{array} \right\} \Rightarrow \sin (\pi - \alpha ) \ne \sin (\pi + \alpha )$. A sai
Đáp án B: $\left. \begin{array}{l}\sin (\dfrac{\pi }{2} - \alpha ) = c{\rm{os}}\alpha \\\sin (\pi + \alpha ) = - \sin \alpha \end{array} \right\} \Rightarrow \sin (\dfrac{\pi }{2} - \alpha ) \ne \sin (\pi + \alpha )$. B sai
Đáp án C: $\left. \begin{array}{l}\sin (\pi + \alpha ) = - \sin \alpha \\\sin ( - \alpha ) = - \sin \alpha \end{array} \right\} \Rightarrow \sin (\pi + \alpha ) = \sin ( - \alpha )$ . C đúng
Đáp án D: $\left. \begin{array}{l}\sin (\pi + \alpha ) = - \sin \alpha \\\cos (\dfrac{\pi }{2} - \alpha ) = \sin \alpha \end{array} \right\} \Rightarrow \sin (\pi + \alpha ) \ne \cos (\dfrac{\pi }{2} - \alpha )$. D sai