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Course Notes for Calculus I

Section 5.1 Limits at Infinity

Subsection 5.1.1 Definitions

In addition to asking what happens to a function as the input approaches paticular finite values, I can also ask what happens as the input ‘approaches infinityi’, that is, as the input gets larger and larger.
The statement
\begin{equation*} \lim_{x \rightarrow \infty} f(x) = L \end{equation*}
means that as \(x\) gets larger and larger without bound, \(f(x)\) gets closer and closer to \(L\text{.}\) The statement
\begin{equation*} \lim_{x \rightarrow \infty} f(x) = \infty \end{equation*}
means that as \(x\) gets larger and larger without bound, \(f(x)\) also gets larger and larger without bound. The statement
\begin{equation*} \lim_{x \rightarrow \infty} f(x) = -\infty \end{equation*}
means that as \(x\) gets larger and larger without bound, \(f(x)\) becomes a larger and larger negative number without bound.
These statements are defined the same way for \(x \rightarrow -\infty\) when the input \(x\) becomes a larger and larger negative number without bound.
In all of these definitions, the symbol \(\infty\) means ‘getting larger and larger without bound’. It is never a number, never a quantity. It is a convenient shorthand for the idea of unbounded growth. Please keep this in mind, particularly for your algebra and arithmetic. Inifnity is not a number and doing algebra or arithmetic with it doesn’t make sense.
Figure 5.1.1. Horizontal Asymptotes

Subsection 5.1.2 Horizontal Asymptotes

When a function \(f(x)\) has a finite limit as the input growths with out bound:
\begin{equation*} \lim_{x \rightarrow \infty} f(x) = L\text{,} \end{equation*}
then the graph of the function approaches the line \(y=L\text{.}\) Such lines are called horizontal asymptotes. Similarly, if the limit is
\begin{equation*} \lim_{x \rightarrow -\infty} f(x) = L\text{,} \end{equation*}
then again the fuction approach the line \(y=L\) and such a line is also called a horizontal asymptote.
To search for horizontal asymptotes, I consider limits of a function as \(x \rightarrow \pm \infty\text{.}\) If such a limit produces a finite result (the number \(L\) above), then the line \(y=L\) is a horizontal asymptote. Note that there are two possibilities for horizontal asymptotes: in the positive and in the negative directions. It is possible for a function to have a horizontal asymptote in only one direction, or in both directions but at different values.