Prove that if \begin{equation}\lim_{x\to a-}\,f(x)\neq \lim_{x\to a+}\,f(x)\end{equation} then \begin{equation}\lim_{x\to a\,}\,f(x)\end{equation} does not exist.

Problem: 

Prove that if \begin{equation}\lim_{x\to a-}\,f(x)\neq \lim_{x\to a+}\,f(x)\end{equation} then \begin{equation}\lim_{x\to a\,}\,f(x)\end{equation} does not exist.

Answer: 

It is true that if \begin{equation}\lim_{x\to a-}\,f(x)\neq \lim_{x\to a+}\,f(x)\end{equation} then \begin{equation}\lim_{x\to a\,}\,f(x)\end{equation} does not exist.