Prove that \[\lim_{h\to 0}\,f(a+h)=L\] if and only if \[\lim_{x\to a}\,f(x)=L.\]

Problem: 

Prove that \[\lim_{h\to 0}\,f(a+h)=L\] if and only if \[\lim_{x\to a}\,f(x)=L.\]

Answer: 

It is true that \[\lim_{h\to 0}\,f(a+h)=L\] if and only if \[\lim_{x\to a}\,f(x)=L.\]