$$\eqalign{
& y = \sqrt {{{1 - \cos 2x} \over {1 + {{\cos }^2}2x}}} \cr
& 1 + {\cos ^2}2x > 0\,\,\forall x \cr
& DKXD:\,\,{{1 - \cos 2x} \over {1 + {{\cos }^2}2x}} \ge 0 \cr
& \Leftrightarrow 1 - \cos 2x \ge 0 \Leftrightarrow \cos 2x \le 1\,\,\left( {luon\,\,dung} \right) \cr
& \Rightarrow TXD:\,\,D = R \cr} $$