Câu hỏi 5

TXĐ: \(D = R\backslash \left\{ {{\pi  \over 2} + k\pi \,\,\left( {k \in Z} \right)} \right\}\)

Ta có: \({\tan ^2}x \ge 0 \Leftrightarrow 1 + {\tan ^2}x \ge 1 \Leftrightarrow {2 \over {1 + {{\tan }^2}x}} \le 2\)

Vậy \(\max y = 2 \Leftrightarrow \tan x = 0 \Leftrightarrow x = k\pi \,\,\left( {k \in Z} \right)\)

Chọn B.