Đặt \(t=x-10\Rightarrow\begin{cases}x=t+10\\x\rightarrow t;t\rightarrow0\end{cases}\)
\(\Rightarrow L=\lim\limits_{t\rightarrow0}\frac{lg\left(t+10\right)-lg10}{t}=\lim\limits_{t\rightarrow0}\frac{lg\left(\frac{t+10}{10}\right)}{t}=\lim\limits_{t\rightarrow0}\left[\frac{lg\left(1+\frac{t}{10}\right)}{\frac{t}{10}}.\frac{1}{10}\right]=\frac{1}{10}\)