17Calculus - One-Sided Limits
17Calculus
One-sided limits require a good understanding of piecewise functions. If you are a little rusty or just need a quick reminder, you can find a complete discussion of piecewise functions on the piecewise functions precalculus page.
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One-sided limits are finite limits where we evaluate the limit from each side of a point individually. The notation we use is
\(\displaystyle{ \lim_{x \to a^-}{f(x)} }\) |
evaluate the limit on the left side of \(a\), i.e. values of \(x < a\) |
\(\displaystyle{ \lim_{x \to a^+}{f(x)} }\) |
evaluate the limit on the right side of \(a\), i.e. values of \(x > a\) |
The negative and positive sign that look like exponents on the finite value \(a\) indicate the side that we are looking at.
One of the reasons we need to look at limits on both sides of some number is when we are determining continuity. As you know from the continuity page, one of the requirements for continuity is that the limit at a point must exist. In order for a limit to exist, the limit from the left must be equal to the limit from the right, i.e. \(\displaystyle{\lim_{x \to a^-}{f(x)} = \lim_{x \to a^+}{f(x)}}\).
Notice that we are NOT saying that the function value must be equal to the limit or even that the function need be defined at \(x=a\), only that the limit be equal on both sides of a.
Here is a great video to build your intuition of one-sided limits. He uses an absolute value function to discuss the idea of one-sided limits and limits that do not exist.
Dr Chris Tisdell - Limit of a function [9min-11secs]
video by Dr Chris Tisdell |
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Here is a good video showing a graph with several one-sided limits.
Krista King Math - How to find limits on CRAZY GRAPHS [7min-47secs]
video by Krista King Math |
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You will see more one-sided limits when you learn about continuity. For now, let's work these practice problems.
Practice
Unless otherwise instructed, evaluate these limits. Give your answers in exact form.
Basic
\(\displaystyle{\lim_{x\to1}{\frac{|x-1|}{x-1}}}\)
Problem Statement
Evaluate the limit, giving your answer in exact form. \(\displaystyle{\lim_{x\to1}{\frac{|x-1|}{x-1}}}\)
Solution
PatrickJMT - 484 video solution
video by PatrickJMT |
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\(\displaystyle{\lim_{x\to0}{\frac{x-1}{x^2(x+2)}}}\)
Problem Statement
Evaluate the limit, giving your answer in exact form. \(\displaystyle{\lim_{x\to0}{\frac{x-1}{x^2(x+2)}}}\)
Solution
PatrickJMT - 487 video solution
video by PatrickJMT |
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\(\displaystyle{\lim_{x\to-4^-}{|x+4|}}\)
Problem Statement
Evaluate the following limit, giving your answer in exact form. \(\displaystyle{\lim_{x\to-4^-}{|x+4|}}\)
Solution
PatrickJMT - 489 video solution
video by PatrickJMT |
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\(\displaystyle{\lim_{x\to5^+}{\sqrt{x^2-25}}}\)
Problem Statement
Evaluate the limit, giving your answer in exact form. \(\displaystyle{\lim_{x\to5^+}{\sqrt{x^2-25}}}\)
Solution
PatrickJMT - 490 video solution
video by PatrickJMT |
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\(\displaystyle{\lim_{x\to5^-}{\sqrt{x(5-x)}}}\)
Problem Statement
Evaluate the limit, giving your answer in exact form. \(\displaystyle{\lim_{x\to5^-}{\sqrt{x(5-x)}}}\)
Solution
Krista King Math - 491 video solution
video by Krista King Math |
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\(\displaystyle{ \lim_{x\to0}{1/x} }\)
Problem Statement |
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Evaluate the limit \(\displaystyle{ \lim_{x\to0}{1/x} }\) giving your answer in exact form.
Final Answer |
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The limit does not exist.
Problem Statement
Evaluate the limit \(\displaystyle{ \lim_{x\to0}{1/x} }\) giving your answer in exact form.
Solution
For this solution, we will look at specific values of \(x\) to give us a feel for what is happening. Although not recommended for most problems, this technique, along with the graph, can give a good understanding of this function around \(x=0\). Of course, we can't just plug in zero for \(x\) since we end up with \(1/0\) which is not valid.
Here is the plot of \(1/x\).
First, let's look at values of \(f(x)=1/x\) for \(x \gt 0\).
\( f(2) = 1/2 \) |
\( f(1) = 1 \) |
\( f(0.1) = 10 \) |
\( f(0.01) = 100 \) |
\( f(0.001) = 1000 \) |
So it looks like as \(x\) gets smaller, \(f(x)\) gets larger. This can be seen in the graph. So on the right side of the graph, \(\displaystyle{ \lim_{x\to0^+}{1/x} = \infty }\).
Now let's look at values of \(f(x)=1/x\) for \(x \lt 0\).
\( f(-2) = -1/2 \) |
\( f(-1) = -1 \) |
\( f(-0.1) = -10 \) |
\( f(-0.01) = -100 \) |
\( f(-0.001) = -1000 \) |
These values are getting smaller, which you can also see in the graph. So on the left side of the graph \(\displaystyle{ \lim_{x\to0^-}{1/x} = -\infty }\).
Since the limits are different on each side of the graph, i.e. \(\displaystyle{ \lim_{x\to0^+}{1/x} \neq \lim_{x\to0^-}{1/x} }\), the limit \(\displaystyle{\lim_{x\to0}{1/x}}\) does not exist.
Final Answer
The limit does not exist.
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\(\displaystyle{ \lim_{x\to0}{1/x^2} }\)
Problem Statement |
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Evaluate the limit \(\displaystyle{ \lim_{x\to0}{1/x^2} }\) giving your answer in exact form.
Final Answer |
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\(\displaystyle{ \lim_{x\to0}{1/x^2} }\) \( = \infty \)
Problem Statement
Evaluate the limit \(\displaystyle{ \lim_{x\to0}{1/x^2} }\) giving your answer in exact form.
Solution
For this solution, we will look at specific values of \(x\) to give us a feel for what is happening. Although not recommended for most problems, this technique, along with the graph, can give a good understanding of this function around \(x=0\). Of course, we can't just plug in zero for \(x\) since we end up with \(1/0\) which is not valid.
Here is the plot of \(1/x^2\).
First, let's look at values of \(f(x)=1/x^2\) for \(x \gt 0\).
\( f(2) = 1/4 \) |
\( f(1) = 1 \) |
\( f(0.1) = 100 \) |
\( f(0.01) = 10000 \) |
\( f(0.001) = 1000000 \) |
So it looks like as \(x\) gets smaller, \(f(x)\) gets larger. This can be seen in the graph. So on the right side of the graph, \(\displaystyle{ \lim_{x\to0^+}{1/x^2} = \infty }\).
Now let's look at values of \(f(x)=1/x\) for \(x \lt 0\).
\( f(-2) = 1/4 \) |
\( f(-1) = 1 \) |
\( f(-0.1) = 100 \) |
\( f(-0.01) = 10000 \) |
\( f(-0.001) = 1000000 \) |
These values are also getting larger, which you can also see in the graph. So on the left side of the graph \(\displaystyle{ \lim_{x\to0^-}{1/x} = \infty }\).
Since \(\displaystyle{ \lim_{x\to0^+}{1/x} = \lim_{x\to0^-}{1/x} }\), the limit \(\displaystyle{\lim_{x\to0}{1/x} = \infty}\).
Final Answer
\(\displaystyle{ \lim_{x\to0}{1/x^2} }\) \( = \infty \)
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Intermediate
Evaluate \(\displaystyle{\lim_{x\to1}{f(x)}}\) for \(\displaystyle{f(x)=\left\{\begin{array}{lr} x+3 & x \leq 1 \\ x^2-2x & x >1 \end{array}\right.}\)
Problem Statement
Evaluate \(\displaystyle{\lim_{x\to1}{f(x)}}\) for \(\displaystyle{f(x)=\left\{\begin{array}{lr} x+3 & x \leq 1 \\ x^2-2x & x >1 \end{array}\right.}\)
Solution
PatrickJMT - 488 video solution
video by PatrickJMT |
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Prove that the limit \(\displaystyle{\lim_{x\to0}{\frac{|x|}{x}}}\) does not exist.
Problem Statement
Prove that the limit \(\displaystyle{\lim_{x\to0}{\frac{|x|}{x}}}\) does not exist.
Solution
Krista King Math - 492 video solution
video by Krista King Math |
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\(\displaystyle{\lim_{x\to0}{\frac{x-2\abs{x}}{\abs{x}}}}\)
Problem Statement |
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Evaluate the limit \(\displaystyle{\lim_{x\to0}{\frac{x-2\abs{x}}{\abs{x}}}}\) giving your answer in exact form.
Final Answer |
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The limit does not exist.
Problem Statement
Evaluate the limit \(\displaystyle{\lim_{x\to0}{\frac{x-2\abs{x}}{\abs{x}}}}\) giving your answer in exact form.
Solution
This is a quite interesting function. First, let's look at the equation. Since the function \(\displaystyle{ f(x) = \frac{x-2\abs{x}}{\abs{x}} }\) contains absolute values, we need to set up a piecewise function.
When \(x \lt 0\), \(\displaystyle{ f(x) = \frac{x+2x}{-x} = -3 }\)
When \(x \gt 0\), \(\displaystyle{ f(x) = \frac{x-2x}{x} = -1 }\)
So our piecewise function is
\(
f(x) = \left\{
\begin{array}{rrl}
-3 & & x \lt 0 \\
-1 & & x \gt 0
\end{array} \right.
\)
Here is the plot of \(f(x)\).
So we can see from the piecewise function and the plot that \(\displaystyle{\lim_{x\to0^+}{f(x)} = -1 }\) and \(\displaystyle{\lim_{x\to0^-}{f(x)} = -3 }\). Since the limits are different on each side of \(x=0\), the limit does not exist.
Final Answer
The limit does not exist.
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Practice Instructions
Unless otherwise instructed, evaluate these limits. Give your answers in exact form.
