Câu hỏi:  

A x=\frac{\Pi }{4}+k\Pik\epsilon Z

B x = -\frac{\Pi }{4}+k\Pik\epsilon Z

C x = \frac{\Pi }{3}+ k\Pik\epsilon Z

D x = -\frac{\Pi }{3}+ k\Pik\epsilon Z