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