For some functions, it is appropriate to look at their behavior from one side only. If x approaches c from the right only, you write
or if x approaches c from the left only, you write
It follows, then, that if and only if
Example 1: Evaluate
Because x is approaching 0 from the right, it is always positive; is getting closer and closer to zero, so . Although substituting 0 for x would yield the same answer, the next example illustrates why this technique is not always appropriate.
Example 2: Evaluate .
Because x is approaching 0 from the left, it is always negative, and does not exist. In this situation, DNE. Also, note that DNE because .
Example 3: Evaluate
a. As x approaches 2 from the left, x − 2 is negative, and | x − 2|=− ( x − 2); hence,
b. As x approaches 2 from the right, x − 2 is positive, and | x − 2|= x − 2; hence;