\[f(x) = \sqrt{x^{2} - 5x + 6};\]
\[f^{'}(x) = (2x - 5) \bullet \frac{1}{2\sqrt{x^{2} - 5x + 6}} =\]
\[= \frac{2x - 5}{2\sqrt{x^{2} - 5x + 6}}.\]
\[Область\ определения:\]
\[x^{2} - 5x + 6 > 0\]
\[D = 25 - 24 = 1\]
\[x_{1} = \frac{5 - 1}{2} = 2;\text{\ \ }\]
\[x_{2} = \frac{5 + 1}{2} = 3;\]
\[(x - 2)(x - 3) > 0\]
\[x < 2;\ \ \ x > 3.\]
\[Ответ:\ \ \frac{2x - 5}{2\sqrt{(x - 2)(x - 3)}}.\]