In order for a limit to exist, both need to exist, and they must be equal to the same value. This is formally stated as follows.
x→climf(x)=L⇕x→c−limf(x)=x→c+limf(x)=L
Consider the following function.
f(x)=x∣x∣
Its graph is shown below.
Note that as
x approaches
0 from the left, the function keeps getting closer to
-1.
x→c−limf(x)=-1
However, when
x approaches
0 from the right, the function keeps getting closer to
1 instead.
x→c+limf(x)=1
Since both one-sided limits do not coincide, the limit does not exist.
x→climf(x) does not exist