Infinite Limits and Asymptotes
Site: | Saylor Academy |
Course: | MA005: Calculus I |
Book: | Infinite Limits and Asymptotes |
Printed by: | Guest user |
Date: | Thursday, May 16, 2024, 11:47 PM |
Description
Read this section to learn how to use and apply infinite limits to asymptotes. Work through practice problems 1-8.
Introduction
When you turn on an automobile or a light bulb many things happen, and some of them are uniquely part of the start up of the system. These "transient" things occur only during start up, and then the system settles down to its steady-state operation. The start up behavior of systems can be very important, but sometimes we want to investigate the steady-state or long term behavior of the system: how is the system behaving "after a long time?" In this section we consider ways of investigating and describing the long term behavior of functions and the systems they may model: how is a function behaving "when (or ) is arbitrarily large?"
Source: Dale Hoffman, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/12/MA005-4.6-Infinite-Limits-and-Asymptotes.pdf
This work is licensed under a Creative Commons Attribution 3.0 License.
Limits As X Becomes Arbitrarily Large
The same type of questions we considered about a function as approached a finite number can also asked about as "becomes arbitrarily large," "increases without bound," and is eventually larger than any fixed number.
Example 1: What happens to the values of (Fig. 1) and as becomes arbitrarily large, as increases without bound?
Solution: One approach is numerical: evaluate and for some "large" values of and see if there is a pattern to the values of and . Fig. 1 shows the values of and for several large values of . When is very large, it appears that the values of are close to and the values of are close to . In fact, we can guarantee that the values of are as close to as someone wants by taking to be "big enough".
Fig. 1
The values of may or may not ever equal (they never do), but if is "large," then is "close to" . Similarly, we can guarantee that the values of are as close to as someone wants by taking to be "big enough". The graphs of and are shown in Fig. 2 for "large" values of .
Fig. 2
Practice 1: What happens to the values of and as becomes arbitrarily large?
The answers for Example 1 can be written as limit statements:
"As becomes arbitrarily large, the values of approach can be written "" and "the values of approach ". can be written " ".
The symbol " " is read "the limit as approaches infinity" and means "the limit as becomes, arbitrarily large" or as increases without bound. (During this discussion and throughout this book, we do not treat "infinity" or "", as a number, but only as a useful notation. "Infinity" is not part of the real number system, and we use the common notation " " and the phrase " approaches infinity" only to mean that " becomes arbitrarily large". The notation " , " read as " approaches negative infinity," means that the values of become arbitrarily large.)
Practice 2: Write your answers to Practice 1 using the limit notation.
The asks about the behavior of as the values of get larger and larger without any bound, and one way to determine this behavior is to look at the values of at some values of which are "large". If the values of the function get arbitrarily close to a single number as gets larger and larger, then we will say that number is the limit of the function as approaches infinity. A definition of the limit as " " is given at the end of this section.
Practice 3: Fill in the table in Fig. 3 for and , and then use those values to estimate and .
Fig. 3
Example 2: How large does need to be to guarantee that (assume )?
Solution: If , then (Fig.4). If , then .
In general, if is any positive number, then we can guarantee that by picking only values of : if , then .
From this we can conclude that .
Fig. 4
Practice 4: How large does need to be to guarantee that (assume )? Evaluate
The Main Limit Theorem (Section 1.2) about limits of combinations of functions is true if the limits as " " are replaced with limits as " ", but we will not prove those results.
Polynomials occur commonly, and we often need the limit, as , of ratios of polynomials or functions containing powers of . In those situations the following technique is often helpful:
(i) factor the highest power of in the denominator from both the numerator and the denominator, and
(ii) cancel the common factor from the numerator and denominator.
The limit of the new denominator is a constant, so the limit of the resulting ratio is easier to determine.
Solutions:
Similarly,
If we have a difficult limit, as , it is often useful to algebraically manipulate the function into the form of a ratio and then use the previous technique.
If the values of the function oscillate and do not approach a single number as becomes arbitrarily large, then the function does not have a limit as approaches infinity: the limit Does Not Exist.
Solution: and do not have limits as . As grows without bound, the values of oscillate between and (Fig. 5), and these values of do not approach a single number. Similarly, continues to take on values between and , and these values are not approaching a single number.
Fig. 5
Using Calculators To Help Find Limits as " " or " "
Calculators only store a limited number of digits of a number, and this is a severe limitation when we are dealing with extremely large numbers.
Example: The value of is clearly equal to for all values of , and your calculator will give the right answer if you use it to evaluate or . Now use it to evaluate for a big value of , say . , but most calculators do not store digits of a number, and they will respond that which is wrong. In this example the calculator's error is obvious, but the same type of errors can occur in less obvious ways when very large numbers are used on calculators.
You need to be careful with and somewhat suspicious of the answers your calculator gives you.
Calculators can still be helpful for examining some limits as " " and " " as long as we do not place too much faith in their responses.
Even if you have forgotten some of the properties of natural logarithm function and the cube root function , a little experimentation on your calculator can help you determine that .
The Limit is Infinite
The function is undefined at , but we can still ask about the behavior of for values of "close to" . Fig. 6 indicates that if is very small, close to , then is very large. As the values of get closer to , the values of grow larger and can be made as large as we want by picking to be close enough to . Even though the values of are not approaching any number, we use the "infinity" notation to indicate that the values of are growing without bound, and write .
Fig. 6
The values of do not equal "infinity:" means that the values of can be made arbitrarily large by picking values of very close to .
The limit, as , of is slightly more complicated. If is close to , then the value of can be a large positive number or a large negative number, depending on the sign of .
The function does not have a (two-sided) limit as approaches , but we can still ask about one-sided limits:
Solution: (a) As , then and . Since the denominator is approaching we cannot use the Main Limit Theorem, and we need to examine the functions more carefully. If , then so . If is close to and slightly larger than , then the ratio of to is the ratio . As gets closer to is . By taking closer to , the denominator gets closer to but is always positive, so the ratio gets arbitrarily large and negative: .
(b) As , then and gets arbitrarily close to , and is negative. The value of the ratio is : .
Horizontal Asymptotes
The limits of , as " " and " ," give us information about horizontal asymptotes of .
Example 6: Find any horizontal asymptotes of .
Solution: so the line is a horizontal asymptote of . The limit, as " , " is also so is the only horizontal asymptote of . The graphs of and are given in Fig. 7. A function may or may not cross its asymptote.
Fig. 7
Vertical Asymptotes
Definition: The vertical line is a vertical asymptote of the graph of if either or both of the one-sided limits, as or , of is infinite.
If our function is the ratio of a polynomial and a polynomial , then the only candidates for vertical asymptotes are the values of where . However, the fact that is not enough to guarantee that the line is a vertical asymptote of ; we also need to evaluate . If and , then the line is a vertical asymptote of . If and , then the line may or may not be a vertical asymptote.
Example 7: Find the vertical asymptotes of and .
Solution: so the only values which make the denominator are and , and these are the only candidates to be vertical asymptotes.
and so and are both vertical asymptotes of
so the only candidates to be vertical asymptotes are and
Other Asymptotes as "x→∞" and "x→–∞"
If the limit of as " " or " " is a constant , then the graph of gets close to the horizontal line , and we said that was a horizontal asymptote of . Some functions, however, approach other lines which are not horizontal.
Example 8: Find all asymptotes of .
Solution: If is a large positive number or a large negative number, then is very close to , and the graph of is very close to the line (Fig. 8). The line is an asymptote of the graph of .
Fig. 8
If is a large positive number, then is positive, and the graph of is slightly above the graph of . If is a large negative number, then is negative, and the graph of will be slightly below the graph of . The piece of never equals so the graph of never crosses or touches the graph of the asymptote .
The graph of also has a vertical asymptote at since and .
Practice 7: Find all asymptotes of .
Some functions even have nonlinear asymptotes, asymptotes which are not straight lines. The graphs of these functions approach some nonlinear function when the values of are arbitrarily large.
Example 9: Find all asymptotes of .
Solution: When is very large, positive or negative, then is very close to , and the graph of is very close to the graph of . The function is a nonlinear asymptote of . The denominator of is never , and has no vertical asymptotes.
Practice 8: Find all asymptotes of .
If can be written as a sum of two other functions, , with , then the graph of is asymptotic to the graph of , and is an asymptote of .
Definition of lim f(x) = K
The following definition states precisely what is meant by the phrase "we can guarantee that the values of are arbitrarily close to by using sufficiently large values of ".
Definition: means for every given , there is a number so that if is larger than then is within units of (equivalently; whenever .)
Solution: Typically we need to do two things. First we need to find a value of , usually depending on . Then we need to show that the value of we found satisfies the conditions of the definition.
(i) Assume that is less than and solve for .
If , then and
(ii) For any , take . (Now we can just reverse the order of the steps in part (i). ) If and , then so .
We have shown that "for every given , there is an " that satisfies the definition.
Practice Answers
Practice 1: As becomes arbitrarily large, the values of approach and the values of approach .
Practice 3: The completed table is shown in Fig. 12.
Fig. 12
Practice 4: If , then .
If , then .
If , then .
Practice 5 :
(a) .
As the values , and so takes small negative values.
Then the values of are large negative values so we represent the limit as " ".
(b) .
As the values of , and so takes small positive values. As the values of .
Then the values of are large positive values so we represent the limit as " ".
(c) .
As , the values of and so we need to do more work. The numerator can be factored and then the rational function can be reduced (since we know ):
Practice 6 :
(a) .
has vertical asymptotes at and .
(b) .
The value is not in the domain of . If , then has a "hole" when and no vertical asymptotes.
Practice 7: .
has a vertical asymptote at .
has no horizontal asymptotes.
so has the linear asymptote .
Practice 8 :
.
is not defined at , so has a vertical asymptote or a "hole" when .
so has a "hole" when .
so has the nonlinear asymptote .