limits and their properties 3 copyright © cengage learning. all rights reserved
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Limits and Their Properties3
Copyright © Cengage Learning. All rights reserved.
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Evaluating Limits Analytically
Copyright © Cengage Learning. All rights reserved.
3.3
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Properties of Limits
We have learned that the limit of f (x) as x approaches c does not depend on the value of f at x = c. It may happen, however, that the limit is precisely f (c). In such cases, the limit can be evaluated by direct substitution. That is,
Such well-behaved functions are continuous at c.
Substitute c for x.
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Properties of Limits
In Example 2, note that the limit (as x → 2) of the polynomial function p(x) = 4x2 + 3 is simply the value of p at
x = 2.
This direct substitution property is valid for all polynomial and rational functions with nonzero denominators.
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A Strategy for Finding Limits
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Example 5 – Solution
Because exists, you can apply Theorem 3.6 to
conclude that f and g have the same limit at x = 1.
Factor.
Divide out like factors.
Apply Theorem 3.6.
Use direct substitution.
Simplify.
cont’d
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Example 5 – Solution
So, for all x-values other than x = 1, the functions f and g agree, as shown in Figure 3.15.
f and g agree at all but one point.Figure 3.15
cont’d
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A Strategy for Finding Limits
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Dividing Out and Rationalizing Techniques
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Dividing Out and Rationalizing Techniques
Two techniques for finding limits analytically are shown in Examples 6 and 7.
The dividing out technique involves dividing out common factors, and the rationalizing technique involves rationalizing the numerator of a fractional expression.
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Example 6 – Dividing Out Technique
Find the limit:
Solution:
Although you are taking the limit of a rational function, you cannot apply Theorem 3.3 because the limit of the denominator is 0.
Direct substitution fails.
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Example 6 – Solution
Because the limit of the numerator is also 0, the numerator and denominator have a common factor of (x + 3).
So, for all x –3 you can divide out this factor to obtain
cont’d
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Example 6 – Solution
Using Theorem 3.6, it follows that
This result is shown graphically in Figure 3.16.
Note that the graph of the function f coincides with the graph of the function g(x) = x – 2, except that the graph of f has a gap at the point (–3, –5).
cont’d
Apply Theorem 3.6.
Use direct substitution.
f is undefined when x = –3
Figure 3.16
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Dividing Out and Rationalizing Techniques
In Example 6, direct substitution produced the meaningless fractional form 0/0.
An expression such as 0/0 is called an indeterminate form because you cannot (from the form alone) determine the limit. When you try to evaluate a limit and encounter this form, remember that you must rewrite the fraction so that the new denominator does not have 0 as its limit.
One way to do this is to divide out common factors, as shown in Example 7. A second way is to rationalize the numerator, as shown in Example 7.
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Example 7 – Rationalizing Technique
Find the limit:
Solution:
By direct substitution, you obtain the indeterminate form
0/0.
Direct substitution fails.
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Example 7 – Solution
In this case, you can rewrite the fraction by rationalizing the numerator.
cont’d
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Example 7 – Solution
Now, using Theorem 3.6, you can evaluate the limit as shown.
cont’d
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Example 7 – Solution
A table or a graph can reinforce your conclusion that the
limit is (See Figure 3.18.)
cont’d
The limit of f (x) as x approaches 0 is
Figure 3.18