High School Math : Finding One-Sided Limits

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #1 : Finding One Sided Limits

Let \(\displaystyle f(x)=\frac{1}{x}\).

Find \(\displaystyle \lim_{x\ \to \0^{-}} \frac{1}{x}\).

Possible Answers:

\(\displaystyle 1\)

The limit does not exist.

\(\displaystyle \infty\)

\(\displaystyle 0\)

\(\displaystyle -\infty\)

Correct answer:

\(\displaystyle -\infty\)

Explanation:

1overx

This is a graph of \(\displaystyle \frac{1}{x}\). We know that \(\displaystyle \frac{1}{0}\) is undefined; therefore, there is no value for \(\displaystyle f(0)\). But as we take a look at the graph, we can see that as \(\displaystyle x\) approaches 0 from the left, \(\displaystyle f(x)\) approaches negative infinity. 

This can be illustrated by thinking of small negative numbers.

\(\displaystyle f(-.1)=-10\)

\(\displaystyle f(-.01)=-100\)

NOTE: Pay attention to one-sided limit specifications, as it is easy to pick the wrong answer choice if you're not careful. 

\(\displaystyle \lim_{x \to \0^+} \frac{1}{x}\) is actually infinity, not negative infinity. 

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