Hàm số y= cănx-2 + căn4-x đồng biến trên khoảng nào?

$\begin{array}{l} y = \sqrt {x - 2} + \sqrt {4 - x} \\ TXD:\,\,\,D = \left[ {2;\,\,4} \right]\\ \Rightarrow y' = \frac{1}{{2\sqrt {x - 2} }} - \frac{1}{{2\sqrt {4 - x} }}\\ HS\,\,\,DB \Leftrightarrow y' \ge 0\\ \Leftrightarrow \frac{1}{{2\sqrt {x - 2} }} - \frac{1}{{2\sqrt {4 - x} }} \ge 0\\ \Leftrightarrow \frac{{\sqrt {4 - x} - \sqrt {x - 2} }}{{2\sqrt {x - 2} .\sqrt {4 - x} }} \ge 0\\ \Leftrightarrow \sqrt {4 - x} - \sqrt {x - 2} \ge 0\\ \Leftrightarrow \sqrt {4 - x} \ge \sqrt {x - 2} \\ \Leftrightarrow 4 - x \ge x - 2\\ \Leftrightarrow 2x \le 6\\ \Leftrightarrow x \le 3.\\ \Rightarrow Ket\,\,hop\,\,voi\,\,\,DK:\,\,\,2 \le x \le 4\\ \Rightarrow Hs\,\,\,DB\,\,\,tren\,\,\,\left[ {2;\,\,3} \right]. \end{array}$