Đáp án:B
Giải thích các bước giải:
$\begin{array}{l}
f\left( x \right) = \ln \left( {2x - 5} \right)\\
\Rightarrow f'\left( x \right) = \frac{{\left( {2x - 5} \right)'}}{{2x - 5}} = \frac{2}{{2x - 5}}\\
Khi\,f'\left( x \right) < 1\\
\Rightarrow \frac{2}{{2x - 5}} < 1\\
\Rightarrow \frac{{2 - \left( {2x - 5} \right)}}{{2x - 5}} < 0\\
\Rightarrow \frac{{7 - 2x}}{{2x - 5}} < 0\\
\Rightarrow \left[ \begin{array}{l}
x > \frac{7}{2}\\
x < \frac{5}{2}
\end{array} \right.\\
\Rightarrow \left( { - \infty ;\frac{5}{2}} \right) \cup \left( {\frac{7}{2}; + \infty } \right)
\end{array}$
