Infinite Limits and Asymptotes
Read this section to learn how to use and apply infinite limits to asymptotes. Work through practice problems 1-8.
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 .