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Calculus Volume 1

4.8 L’Hôpital’s Rule

Calculus Volume 14.8 L’Hôpital’s Rule
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  1. Preface
  2. 1 Functions and Graphs
    1. Introduction
    2. 1.1 Review of Functions
    3. 1.2 Basic Classes of Functions
    4. 1.3 Trigonometric Functions
    5. 1.4 Inverse Functions
    6. 1.5 Exponential and Logarithmic Functions
    7. Key Terms
    8. Key Equations
    9. Key Concepts
    10. Chapter Review Exercises
  3. 2 Limits
    1. Introduction
    2. 2.1 A Preview of Calculus
    3. 2.2 The Limit of a Function
    4. 2.3 The Limit Laws
    5. 2.4 Continuity
    6. 2.5 The Precise Definition of a Limit
    7. Key Terms
    8. Key Equations
    9. Key Concepts
    10. Chapter Review Exercises
  4. 3 Derivatives
    1. Introduction
    2. 3.1 Defining the Derivative
    3. 3.2 The Derivative as a Function
    4. 3.3 Differentiation Rules
    5. 3.4 Derivatives as Rates of Change
    6. 3.5 Derivatives of Trigonometric Functions
    7. 3.6 The Chain Rule
    8. 3.7 Derivatives of Inverse Functions
    9. 3.8 Implicit Differentiation
    10. 3.9 Derivatives of Exponential and Logarithmic Functions
    11. Key Terms
    12. Key Equations
    13. Key Concepts
    14. Chapter Review Exercises
  5. 4 Applications of Derivatives
    1. Introduction
    2. 4.1 Related Rates
    3. 4.2 Linear Approximations and Differentials
    4. 4.3 Maxima and Minima
    5. 4.4 The Mean Value Theorem
    6. 4.5 Derivatives and the Shape of a Graph
    7. 4.6 Limits at Infinity and Asymptotes
    8. 4.7 Applied Optimization Problems
    9. 4.8 L’Hôpital’s Rule
    10. 4.9 Newton’s Method
    11. 4.10 Antiderivatives
    12. Key Terms
    13. Key Equations
    14. Key Concepts
    15. Chapter Review Exercises
  6. 5 Integration
    1. Introduction
    2. 5.1 Approximating Areas
    3. 5.2 The Definite Integral
    4. 5.3 The Fundamental Theorem of Calculus
    5. 5.4 Integration Formulas and the Net Change Theorem
    6. 5.5 Substitution
    7. 5.6 Integrals Involving Exponential and Logarithmic Functions
    8. 5.7 Integrals Resulting in Inverse Trigonometric Functions
    9. Key Terms
    10. Key Equations
    11. Key Concepts
    12. Chapter Review Exercises
  7. 6 Applications of Integration
    1. Introduction
    2. 6.1 Areas between Curves
    3. 6.2 Determining Volumes by Slicing
    4. 6.3 Volumes of Revolution: Cylindrical Shells
    5. 6.4 Arc Length of a Curve and Surface Area
    6. 6.5 Physical Applications
    7. 6.6 Moments and Centers of Mass
    8. 6.7 Integrals, Exponential Functions, and Logarithms
    9. 6.8 Exponential Growth and Decay
    10. 6.9 Calculus of the Hyperbolic Functions
    11. Key Terms
    12. Key Equations
    13. Key Concepts
    14. Chapter Review Exercises
  8. A | Table of Integrals
  9. B | Table of Derivatives
  10. C | Review of Pre-Calculus
  11. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
  12. Index

Learning Objectives

  • 4.8.1. Recognize when to apply L’Hôpital’s rule.
  • 4.8.2. Identify indeterminate forms produced by quotients, products, subtractions, and powers, and apply L’Hôpital’s rule in each case.
  • 4.8.3. Describe the relative growth rates of functions.

In this section, we examine a powerful tool for evaluating limits. This tool, known as L’Hôpital’s rule, uses derivatives to calculate limits. With this rule, we will be able to evaluate many limits we have not yet been able to determine. Instead of relying on numerical evidence to conjecture that a limit exists, we will be able to show definitively that a limit exists and to determine its exact value.

Applying L’Hôpital’s Rule

L’Hôpital’s rule can be used to evaluate limits involving the quotient of two functions. Consider

limxaf(x)g(x).limxaf(x)g(x).

If limxaf(x)=L1andlimxag(x)=L20,limxaf(x)=L1andlimxag(x)=L20, then

limxaf(x)g(x)=L1L2.limxaf(x)g(x)=L1L2.

However, what happens if limxaf(x)=0limxaf(x)=0 and limxag(x)=0?limxag(x)=0? We call this one of the indeterminate forms, of type 00.00. This is considered an indeterminate form because we cannot determine the exact behavior of f(x)g(x)f(x)g(x) as xaxa without further analysis. We have seen examples of this earlier in the text. For example, consider

limx2x24x2andlimx0sinxx.limx2x24x2andlimx0sinxx.

For the first of these examples, we can evaluate the limit by factoring the numerator and writing

limx2x24x2=limx2(x+2)(x2)x2=limx2(x+2)=2+2=4.limx2x24x2=limx2(x+2)(x2)x2=limx2(x+2)=2+2=4.

For limx0sinxxlimx0sinxx we were able to show, using a geometric argument, that

limx0sinxx=1.limx0sinxx=1.

Here we use a different technique for evaluating limits such as these. Not only does this technique provide an easier way to evaluate these limits, but also, and more important, it provides us with a way to evaluate many other limits that we could not calculate previously.

The idea behind L’Hôpital’s rule can be explained using local linear approximations. Consider two differentiable functions ff and gg such that limxaf(x)=0=limxag(x)limxaf(x)=0=limxag(x) and such that g(a)0g(a)0 For xx near a,a, we can write

f(x)f(a)+f(a)(xa)f(x)f(a)+f(a)(xa)

and

g(x)g(a)+g(a)(xa).g(x)g(a)+g(a)(xa).

Therefore,

f(x)g(x)f(a)+f(a)(xa)g(a)+g(a)(xa).f(x)g(x)f(a)+f(a)(xa)g(a)+g(a)(xa).
Two functions y = f(x) and y = g(x) are drawn such that they cross at a point above x = a. The linear approximations of these two functions y = f(a) + f’(a)(x – a) and y = g(a) + g’(a)(x – a) are also drawn.
Figure 4.71 If limxaf(x)=limxag(x),limxaf(x)=limxag(x), then the ratio f(x)/g(x)f(x)/g(x) is approximately equal to the ratio of their linear approximations near a.a.

Since ff is differentiable at a,a, then ff is continuous at a,a, and therefore f(a)=limxaf(x)=0.f(a)=limxaf(x)=0. Similarly, g(a)=limxag(x)=0.g(a)=limxag(x)=0. If we also assume that ff and gg are continuous at x=a,x=a, then f(a)=limxaf(x)f(a)=limxaf(x) and g(a)=limxag(x).g(a)=limxag(x). Using these ideas, we conclude that

limxaf(x)g(x)=limxaf(x)(xa)g(x)(xa)=limxaf(x)g(x).limxaf(x)g(x)=limxaf(x)(xa)g(x)(xa)=limxaf(x)g(x).

Note that the assumption that ff and gg are continuous at aa and g(a)0g(a)0 can be loosened. We state L’Hôpital’s rule formally for the indeterminate form 00.00. Also note that the notation 0000 does not mean we are actually dividing zero by zero. Rather, we are using the notation 0000 to represent a quotient of limits, each of which is zero.

Theorem 4.12

L’Hôpital’s Rule (0/0 Case)

Suppose ff and gg are differentiable functions over an open interval containing a,a, except possibly at a.a. If limxaf(x)=0limxaf(x)=0 and limxag(x)=0,limxag(x)=0, then

limxaf(x)g(x)=limxaf(x)g(x),limxaf(x)g(x)=limxaf(x)g(x),

assuming the limit on the right exists or is or .. This result also holds if we are considering one-sided limits, or if a=and.a=and.

Proof

We provide a proof of this theorem in the special case when f,g,f,f,g,f, and gg are all continuous over an open interval containing a.a. In that case, since limxaf(x)=0=limxag(x)limxaf(x)=0=limxag(x) and ff and gg are continuous at a,a, it follows that f(a)=0=g(a).f(a)=0=g(a). Therefore,

limxaf(x)g(x)=limxaf(x)f(a)g(x)g(a)sincef(a)=0=g(a)=limxaf(x)f(a)xag(x)g(a)xaalgebra=limxaf(x)f(a)xalimxag(x)g(a)xalimit of a quotient=f(a)g(a)definition of the derivative=limxaf(x)limxag(x)continuity offandg=limxaf(x)g(x).limit of a quotientlimxaf(x)g(x)=limxaf(x)f(a)g(x)g(a)sincef(a)=0=g(a)=limxaf(x)f(a)xag(x)g(a)xaalgebra=limxaf(x)f(a)xalimxag(x)g(a)xalimit of a quotient=f(a)g(a)definition of the derivative=limxaf(x)limxag(x)continuity offandg=limxaf(x)g(x).limit of a quotient

Note that L’Hôpital’s rule states we can calculate the limit of a quotient fgfg by considering the limit of the quotient of the derivatives fg.fg. It is important to realize that we are not calculating the derivative of the quotient fg.fg.

Example 4.38

Applying L’Hôpital’s Rule (0/0 Case)

Evaluate each of the following limits by applying L’Hôpital’s rule.

  1. limx01cosxxlimx01cosxx
  2. limx1sin(πx)lnxlimx1sin(πx)lnx
  3. limxe1/x11/xlimxe1/x11/x
  4. limx0sinxxx2limx0sinxxx2

Solution

  1. Since the numerator 1cosx01cosx0 and the denominator x0,x0, we can apply L’Hôpital’s rule to evaluate this limit. We have
    limx01cosxx=limx0ddx(1cosx)ddx(x)=limx0sinx1=limx0(sinx)limx0(1)=01=0.limx01cosxx=limx0ddx(1cosx)ddx(x)=limx0sinx1=limx0(sinx)limx0(1)=01=0.
  2. As x1,x1, the numerator sin(πx)0sin(πx)0 and the denominator ln(x)0.ln(x)0. Therefore, we can apply L’Hôpital’s rule. We obtain
    limx1sin(πx)lnx=limx1πcos(πx)1/x=limx1(πx)cos(πx)=(π·1)(−1)=π.limx1sin(πx)lnx=limx1πcos(πx)1/x=limx1(πx)cos(πx)=(π·1)(−1)=π.
  3. As x,x, the numerator e1/x10e1/x10 and the denominator (1x)0.(1x)0. Therefore, we can apply L’Hôpital’s rule. We obtain
    limxe1/x11x=limxe1/x(−1x2)(−1x2)=limxe1/x=e0=1.limxe1/x11x=limxe1/x(−1x2)(−1x2)=limxe1/x=e0=1.
  4. As x0,x0, both the numerator and denominator approach zero. Therefore, we can apply L’Hôpital’s rule. We obtain
    limx0sinxxx2=limx0cosx12x.limx0sinxxx2=limx0cosx12x.

    Since the numerator and denominator of this new quotient both approach zero as x0,x0, we apply L’Hôpital’s rule again. In doing so, we see that
    limx0cosx12x=limx0sinx2=0.limx0cosx12x=limx0sinx2=0.

    Therefore, we conclude that
    limx0sinxxx2=0.limx0sinxxx2=0.

Checkpoint 4.37

Evaluate limx0xtanx.limx0xtanx.

We can also use L’Hôpital’s rule to evaluate limits of quotients f(x)g(x)f(x)g(x) in which f(x)±f(x)± and g(x)±.g(x)±. Limits of this form are classified as indeterminate forms of type /./. Again, note that we are not actually dividing by .. Since is not a real number, that is impossible; rather, /./. is used to represent a quotient of limits, each of which is or ..

Theorem 4.13

L’Hôpital’s Rule (/(/ Case)

Suppose ff and gg are differentiable functions over an open interval containing a,a, except possibly at a.a. Suppose limxaf(x)=limxaf(x)= (or )) and limxag(x)=limxag(x)= (or ).). Then,

limxaf(x)g(x)=limxaf(x)g(x),limxaf(x)g(x)=limxaf(x)g(x),

assuming the limit on the right exists or is or .. This result also holds if the limit is infinite, if a=a= or ,, or the limit is one-sided.

Example 4.39

Applying L’Hôpital’s Rule (/(/ Case)

Evaluate each of the following limits by applying L’Hôpital’s rule.

  1. limx3x+52x+1limx3x+52x+1
  2. limx0+lnxcotxlimx0+lnxcotx

Solution

  1. Since 3x+53x+5 and 2x+12x+1 are first-degree polynomials with positive leading coefficients, limx(3x+5)=limx(3x+5)= and limx(2x+1)=.limx(2x+1)=. Therefore, we apply L’Hôpital’s rule and obtain
    limx3x+52x+1/x=limx32=32.limx3x+52x+1/x=limx32=32.

    Note that this limit can also be calculated without invoking L’Hôpital’s rule. Earlier in the chapter we showed how to evaluate such a limit by dividing the numerator and denominator by the highest power of xx in the denominator. In doing so, we saw that
    limx3x+52x+1=limx3+5/x2x+1/x=32.limx3x+52x+1=limx3+5/x2x+1/x=32.

    L’Hôpital’s rule provides us with an alternative means of evaluating this type of limit.
  2. Here, limx0+lnx=limx0+lnx= and limx0+cotx=.limx0+cotx=. Therefore, we can apply L’Hôpital’s rule and obtain
    limx0+lnxcotx=limx0+1/xcsc2x=limx0+1xcsc2x.limx0+lnxcotx=limx0+1/xcsc2x=limx0+1xcsc2x.

    Now as x0+,x0+, csc2x.csc2x. Therefore, the first term in the denominator is approaching zero and the second term is getting really large. In such a case, anything can happen with the product. Therefore, we cannot make any conclusion yet. To evaluate the limit, we use the definition of cscxcscx to write
    limx0+1xcsc2x=limx0+sin2xx.limx0+1xcsc2x=limx0+sin2xx.

    Now limx0+sin2x=0limx0+sin2x=0 and limx0+x=0,limx0+x=0, so we apply L’Hôpital’s rule again. We find
    limx0+sin2xx=limx0+2sinxcosx−1=0−1=0.limx0+sin2xx=limx0+2sinxcosx−1=0−1=0.

    We conclude that
    limx0+lnxcotx=0.limx0+lnxcotx=0.

Checkpoint 4.38

Evaluate limxlnx5x.limxlnx5x.

As mentioned, L’Hôpital’s rule is an extremely useful tool for evaluating limits. It is important to remember, however, that to apply L’Hôpital’s rule to a quotient f(x)g(x),f(x)g(x), it is essential that the limit of f(x)g(x)f(x)g(x) be of the form 0000 or /./. Consider the following example.

Example 4.40

When L’Hôpital’s Rule Does Not Apply

Consider limx1x2+53x+4.limx1x2+53x+4. Show that the limit cannot be evaluated by applying L’Hôpital’s rule.

Solution

Because the limits of the numerator and denominator are not both zero and are not both infinite, we cannot apply L’Hôpital’s rule. If we try to do so, we get

ddx(x2+5)=2xddx(x2+5)=2x

and

ddx(3x+4)=3.ddx(3x+4)=3.

At which point we would conclude erroneously that

limx1x2+53x+4=limx12x3=23.limx1x2+53x+4=limx12x3=23.

However, since limx1(x2+5)=6limx1(x2+5)=6 and limx1(3x+4)=7,limx1(3x+4)=7, we actually have

limx1x2+53x+4=67.limx1x2+53x+4=67.

We can conclude that

limx1x2+53x+4limx1ddx(x2+5)ddx(3x+4).limx1x2+53x+4limx1ddx(x2+5)ddx(3x+4).

Checkpoint 4.39

Explain why we cannot apply L’Hôpital’s rule to evaluate limx0+cosxx.limx0+cosxx. Evaluate limx0+cosxxlimx0+cosxx by other means.

Other Indeterminate Forms

L’Hôpital’s rule is very useful for evaluating limits involving the indeterminate forms 0000 and /./. However, we can also use L’Hôpital’s rule to help evaluate limits involving other indeterminate forms that arise when evaluating limits. The expressions 0·,0·, ,, 1,1, 0,0, and 0000 are all considered indeterminate forms. These expressions are not real numbers. Rather, they represent forms that arise when trying to evaluate certain limits. Next we realize why these are indeterminate forms and then understand how to use L’Hôpital’s rule in these cases. The key idea is that we must rewrite the indeterminate forms in such a way that we arrive at the indeterminate form 0000 or /./.

Indeterminate Form of Type 0·0·

Suppose we want to evaluate limxa(f(x)·g(x)),limxa(f(x)·g(x)), where f(x)0f(x)0 and g(x)g(x) (or )) as xa.xa. Since one term in the product is approaching zero but the other term is becoming arbitrarily large (in magnitude), anything can happen to the product. We use the notation 0·0· to denote the form that arises in this situation. The expression 0·0· is considered indeterminate because we cannot determine without further analysis the exact behavior of the product f(x)g(x)f(x)g(x) as xa.xa. For example, let nn be a positive integer and consider

f(x)=1(xn+1)andg(x)=3x2.f(x)=1(xn+1)andg(x)=3x2.

As x,x, f(x)0f(x)0 and g(x).g(x). However, the limit as xx of f(x)g(x)=3x2(xn+1)f(x)g(x)=3x2(xn+1) varies, depending on n.n. If n=2,n=2, then limxf(x)g(x)=3.limxf(x)g(x)=3. If n=1,n=1, then limxf(x)g(x)=.limxf(x)g(x)=. If n=3,n=3, then limxf(x)g(x)=0.limxf(x)g(x)=0. Here we consider another limit involving the indeterminate form 0·0· and show how to rewrite the function as a quotient to use L’Hôpital’s rule.

Example 4.41

Indeterminate Form of Type 0·0·

Evaluate limx0+xlnx.limx0+xlnx.

Solution

First, rewrite the function xlnxxlnx as a quotient to apply L’Hôpital’s rule. If we write

xlnx=lnx1/x,xlnx=lnx1/x,

we see that lnxlnx as x0+x0+ and 1x1x as x0+.x0+. Therefore, we can apply L’Hôpital’s rule and obtain

limx0+lnx1/x=limx0+ddx(lnx)ddx(1/x)=limx0+1/x-1/x2=limx0+(x)=0.limx0+lnx1/x=limx0+ddx(lnx)ddx(1/x)=limx0+1/x-1/x2=limx0+(x)=0.

We conclude that

limx0+xlnx=0.limx0+xlnx=0.
The function y = x ln(x) is graphed for values x ≥ 0. At x = 0, the value of the function is 0.
Figure 4.72 Finding the limit at x=0x=0 of the function f(x)=xlnx.f(x)=xlnx.

Checkpoint 4.40

Evaluate limx0xcotx.limx0xcotx.

Indeterminate Form of Type

Another type of indeterminate form is .. Consider the following example. Let nn be a positive integer and let f(x)=3xnf(x)=3xn and g(x)=3x2+5.g(x)=3x2+5. As x,x, f(x)f(x) and g(x).g(x). We are interested in limx(f(x)g(x)).limx(f(x)g(x)). Depending on whether f(x)f(x) grows faster, g(x)g(x) grows faster, or they grow at the same rate, as we see next, anything can happen in this limit. Since f(x)f(x) and g(x),g(x), we write to denote the form of this limit. As with our other indeterminate forms, has no meaning on its own and we must do more analysis to determine the value of the limit. For example, suppose the exponent nn in the function f(x)=3xnf(x)=3xn is n=3,n=3, then

limx(f(x)g(x))=limx(3x33x25)=.limx(f(x)g(x))=limx(3x33x25)=.

On the other hand, if n=2,n=2, then

limx(f(x)g(x))=limx(3x23x25)=−5.limx(f(x)g(x))=limx(3x23x25)=−5.

However, if n=1,n=1, then

limx(f(x)g(x))=limx(3x3x25)=.limx(f(x)g(x))=limx(3x3x25)=.

Therefore, the limit cannot be determined by considering only .. Next we see how to rewrite an expression involving the indeterminate form as a fraction to apply L’Hôpital’s rule.

Example 4.42

Indeterminate Form of Type

Evaluate limx0+(1x21tanx).limx0+(1x21tanx).

Solution

By combining the fractions, we can write the function as a quotient. Since the least common denominator is x2tanx,x2tanx, we have

1x21tanx=(tanx)x2x2tanx.1x21tanx=(tanx)x2x2tanx.

As x0+,x0+, the numerator tanxx20tanxx20 and the denominator x2tanx0.x2tanx0. Therefore, we can apply L’Hôpital’s rule. Taking the derivatives of the numerator and the denominator, we have

limx0+(tanx)x2x2tanx=limx0+(sec2x)2xx2sec2x+2xtanx.limx0+(tanx)x2x2tanx=limx0+(sec2x)2xx2sec2x+2xtanx.

As x0+,x0+, (sec2x)2x1(sec2x)2x1 and x2sec2x+2xtanx0.x2sec2x+2xtanx0. Since the denominator is positive as xx approaches zero from the right, we conclude that

limx0+(sec2x)2xx2sec2x+2xtanx=.limx0+(sec2x)2xx2sec2x+2xtanx=.

Therefore,

limx0+(1x21tanx)=.limx0+(1x21tanx)=.

Checkpoint 4.41

Evaluate limx0+(1x1sinx).limx0+(1x1sinx).

Another type of indeterminate form that arises when evaluating limits involves exponents. The expressions 00,00, 0,0, and 11 are all indeterminate forms. On their own, these expressions are meaningless because we cannot actually evaluate these expressions as we would evaluate an expression involving real numbers. Rather, these expressions represent forms that arise when finding limits. Now we examine how L’Hôpital’s rule can be used to evaluate limits involving these indeterminate forms.

Since L’Hôpital’s rule applies to quotients, we use the natural logarithm function and its properties to reduce a problem evaluating a limit involving exponents to a related problem involving a limit of a quotient. For example, suppose we want to evaluate limxaf(x)g(x)limxaf(x)g(x) and we arrive at the indeterminate form 0.0. (The indeterminate forms 0000 and 11 can be handled similarly.) We proceed as follows. Let

y=f(x)g(x).y=f(x)g(x).

Then,

lny=ln(f(x)g(x))=g(x)ln(f(x)).lny=ln(f(x)g(x))=g(x)ln(f(x)).

Therefore,

limxa[ln(y)]=limxa[g(x)ln(f(x))].limxa[ln(y)]=limxa[g(x)ln(f(x))].

Since limxaf(x)=,limxaf(x)=, we know that limxaln(f(x))=.limxaln(f(x))=. Therefore, limxag(x)ln(f(x))limxag(x)ln(f(x)) is of the indeterminate form 0·,0·, and we can use the techniques discussed earlier to rewrite the expression g(x)ln(f(x))g(x)ln(f(x)) in a form so that we can apply L’Hôpital’s rule. Suppose limxag(x)ln(f(x))=L,limxag(x)ln(f(x))=L, where LL may be or .. Then

limxa[ln(y)]=L.limxa[ln(y)]=L.

Since the natural logarithm function is continuous, we conclude that

ln(limxay)=L,ln(limxay)=L,

which gives us

limxay=limxaf(x)g(x)=eL.limxay=limxaf(x)g(x)=eL.

Example 4.43

Indeterminate Form of Type 00

Evaluate limxx1/x.limxx1/x.

Solution

Let y=x1/x.y=x1/x. Then,

ln(x1/x)=1xlnx=lnxx.ln(x1/x)=1xlnx=lnxx.

We need to evaluate limxlnxx.limxlnxx. Applying L’Hôpital’s rule, we obtain

limxlny=limxlnxx=limx1/x1=0.limxlny=limxlnxx=limx1/x1=0.

Therefore, limxlny=0.limxlny=0. Since the natural logarithm function is continuous, we conclude that

ln(limxy)=0,ln(limxy)=0,

which leads to

limxy=limxlnxx=e0=1.limxy=limxlnxx=e0=1.

Hence,

limxx1/x=1.limxx1/x=1.

Checkpoint 4.42

Evaluate limxx1/ln(x).limxx1/ln(x).

Example 4.44

Indeterminate Form of Type 0000

Evaluate limx0+xsinx.limx0+xsinx.

Solution

Let

y=xsinx.y=xsinx.

Therefore,

lny=ln(xsinx)=sinxlnx.lny=ln(xsinx)=sinxlnx.

We now evaluate limx0+sinxlnx.limx0+sinxlnx. Since limx0+sinx=0limx0+sinx=0 and limx0+lnx=,limx0+lnx=, we have the indeterminate form 0·.0·. To apply L’Hôpital’s rule, we need to rewrite sinxlnxsinxlnx as a fraction. We could write

sinxlnx=sinx1/lnxsinxlnx=sinx1/lnx

or

sinxlnx=lnx1/sinx=lnxcscx.sinxlnx=lnx1/sinx=lnxcscx.

Let’s consider the first option. In this case, applying L’Hôpital’s rule, we would obtain

limx0+sinxlnx=limx0+sinx1/lnx=limx0+cosx−1/(x(lnx)2)=limx0+(x(lnx)2cosx).limx0+sinxlnx=limx0+sinx1/lnx=limx0+cosx−1/(x(lnx)2)=limx0+(x(lnx)2cosx).

Unfortunately, we not only have another expression involving the indeterminate form 0·,0·, but the new limit is even more complicated to evaluate than the one with which we started. Instead, we try the second option. By writing

sinxlnx=lnx1/sinx=lnxcscx,sinxlnx=lnx1/sinx=lnxcscx,

and applying L’Hôpital’s rule, we obtain

limx0+sinxlnx=limx0+lnxcscx=limx0+1/xcscxcotx=limx0+−1xcscxcotx.limx0+sinxlnx=limx0+lnxcscx=limx0+1/xcscxcotx=limx0+−1xcscxcotx.

Using the fact that cscx=1sinxcscx=1sinx and cotx=cosxsinx,cotx=cosxsinx, we can rewrite the expression on the right-hand side as

limx0+sin2xxcosx=limx0+[sinxx·(tanx)]=(limx0+sinxx)·(limx0+(tanx))=1·0=0.limx0+sin2xxcosx=limx0+[sinxx·(tanx)]=(limx0+sinxx)·(limx0+(tanx))=1·0=0.

We conclude that limx0+lny=0.limx0+lny=0. Therefore, ln(limx0+y)=0ln(limx0+y)=0 and we have

limx0+y=limx0+xsinx=e0=1.limx0+y=limx0+xsinx=e0=1.

Hence,

limx0+xsinx=1.limx0+xsinx=1.

Checkpoint 4.43

Evaluate limx0+xx.limx0+xx.

Growth Rates of Functions

Suppose the functions ff and gg both approach infinity as x.x. Although the values of both functions become arbitrarily large as the values of xx become sufficiently large, sometimes one function is growing more quickly than the other. For example, f(x)=x2f(x)=x2 and g(x)=x3g(x)=x3 both approach infinity as x.x. However, as shown in the following table, the values of x3x3 are growing much faster than the values of x2.x2.

xx 1010 100100 10001000 10,00010,000
f(x)=x2f(x)=x2 100100 10,00010,000 1,000,0001,000,000 100,000,000100,000,000
g(x)=x3g(x)=x3 10001000 1,000,0001,000,000 1,000,000,0001,000,000,000 1,000,000,000,0001,000,000,000,000
Table 4.7 Comparing the Growth Rates of x2x2 and x3x3

In fact,

limxx3x2=limxx=.or, equivalently,limxx2x3=limx1x=0.limxx3x2=limxx=.or, equivalently,limxx2x3=limx1x=0.

As a result, we say x3x3 is growing more rapidly than x2x2 as x.x. On the other hand, for f(x)=x2f(x)=x2 and g(x)=3x2+4x+1,g(x)=3x2+4x+1, although the values of g(x)g(x) are always greater than the values of f(x)f(x) for x>0,x>0, each value of g(x)g(x) is roughly three times the corresponding value of f(x)f(x) as x,x, as shown in the following table. In fact,

limxx23x2+4x+1=13.limxx23x2+4x+1=13.
xx 1010 100100 10001000 10,00010,000
f(x)=x2f(x)=x2 100100 10,00010,000 1,000,0001,000,000 100,000,000100,000,000
g(x)=3x2+4x+1g(x)=3x2+4x+1 341341 30,40130,401 3,004,0013,004,001 300,040,001300,040,001
Table 4.8 Comparing the Growth Rates of x2x2 and 3x2+4x+13x2+4x+1

In this case, we say that x2x2 and 3x2+4x+13x2+4x+1 are growing at the same rate as x.x.

More generally, suppose ff and gg are two functions that approach infinity as x.x. We say gg grows more rapidly than ff as xx if

limxg(x)f(x)=;or, equivalently,limxf(x)g(x)=0.limxg(x)f(x)=;or, equivalently,limxf(x)g(x)=0.

On the other hand, if there exists a constant M0M0 such that

limxf(x)g(x)=M,limxf(x)g(x)=M,

we say ff and gg grow at the same rate as x.x.

Next we see how to use L’Hôpital’s rule to compare the growth rates of power, exponential, and logarithmic functions.

Example 4.45

Comparing the Growth Rates of ln(x),ln(x), x2,x2, and exex

For each of the following pairs of functions, use L’Hôpital’s rule to evaluate limx(f(x)g(x)).limx(f(x)g(x)).

  1. f(x)=x2andg(x)=exf(x)=x2andg(x)=ex
  2. f(x)=ln(x)andg(x)=x2f(x)=ln(x)andg(x)=x2

Solution

  1. Since limxx2=limxx2= and limxex=,limxex=, we can use L’Hôpital’s rule to evaluate limx[x2ex].limx[x2ex]. We obtain
    limxx2ex=limx2xex.limxx2ex=limx2xex.

    Since limx2x=limx2x= and limxex=,limxex=, we can apply L’Hôpital’s rule again. Since
    limx2xex=limx2ex=0,limx2xex=limx2ex=0,

    we conclude that
    limxx2ex=0.limxx2ex=0.

    Therefore, exex grows more rapidly than x2x2 as xx (See Figure 4.73 and Table 4.9).
    The functions g(x) = ex and f(x) = x2 are graphed. It is obvious that g(x) increases much more quickly than f(x).
    Figure 4.73 An exponential function grows at a faster rate than a power function.
    xx 55 1010 1515 2020
    x2x2 2525 100100 225225 400400
    exex 148148 22,02622,026 3,269,0173,269,017 485,165,195485,165,195
    Table 4.9 Growth rates of a power function and an exponential function.
  2. Since limxlnx=limxlnx= and limxx2=,limxx2=, we can use L’Hôpital’s rule to evaluate limxlnxx2.limxlnxx2. We obtain
    limxlnxx2=limx1/x2x=limx12x2=0.limxlnxx2=limx1/x2x=limx12x2=0.

    Thus, x2x2 grows more rapidly than lnxlnx as xx (see Figure 4.74 and Table 4.10).
    The functions g(x) = x2 and f(x) = ln(x) are graphed. It is obvious that g(x) increases much more quickly than f(x).
    Figure 4.74 A power function grows at a faster rate than a logarithmic function.
    xx 1010 100100 10001000 10,00010,000
    ln(x)ln(x) 2.3032.303 4.6054.605 6.9086.908 9.2109.210
    x2x2 100100 10,00010,000 1,000,0001,000,000 100,000,000100,000,000
    Table 4.10 Growth rates of a power function and a logarithmic function
Checkpoint 4.44

Compare the growth rates of x100x100 and 2x.2x.

Using the same ideas as in Example 4.45a. it is not difficult to show that exex grows more rapidly than xpxp for any p>0.p>0. In Figure 4.75 and Table 4.11, we compare exex with x3x3 and x4x4 as x.x.

This figure has two figures marked a and b. In figure a, the functions y = ex and y = x3 are graphed. It is obvious that ex increases more quickly than x3. In figure b, the functions y = ex and y = x4 are graphed. It is obvious that ex increases much more quickly than x4, but the point at which that happens is further to the right than it was for x3.
Figure 4.75 The exponential function exex grows faster than xpxp for any p>0.p>0. (a) A comparison of exex with x3.x3. (b) A comparison of exex with x4.x4.
xx 55 1010 1515 2020
x3x3 125125 10001000 33753375 80008000
x4x4 625625 10,00010,000 50,62550,625 160,000160,000
exex 148148 22,02622,026 3,269,0173,269,017 485,165,195485,165,195
Table 4.11 An exponential function grows at a faster rate than any power function

Similarly, it is not difficult to show that xpxp grows more rapidly than lnxlnx for any p>0.p>0. In Figure 4.76 and Table 4.12, we compare lnxlnx with x3x3 and x.x.

This figure shows y = the square root of x, y = the cube root of x, and y = ln(x). It is apparent that y = ln(x) grows more slowly than either of these functions.
Figure 4.76 The function y=ln(x)y=ln(x) grows more slowly than xpxp for any p>0p>0 as x.x.
xx 1010 100100 10001000 10,00010,000
ln(x)ln(x) 2.3032.303 4.6054.605 6.9086.908 9.2109.210
x3x3 2.1542.154 4.6424.642 1010 21.54421.544
xx 3.1623.162 1010 31.62331.623 100100
Table 4.12 A logarithmic function grows at a slower rate than any root function

Section 4.8 Exercises

For the following exercises, evaluate the limit.

356.

Evaluate the limit limxexx.limxexx.

357.

Evaluate the limit limxexxk.limxexxk.

358.

Evaluate the limit limxlnxxk.limxlnxxk.

359.

Evaluate the limit limxaxax2a2,a0limxaxax2a2,a0.

360.

Evaluate the limit limxaxax3a3,a0limxaxax3a3,a0.

361.

Evaluate the limit limxaxaxnan,a0limxaxaxnan,a0.

For the following exercises, determine whether you can apply L’Hôpital’s rule directly. Explain why or why not. Then, indicate if there is some way you can alter the limit so you can apply L’Hôpital’s rule.

362.

limx0+x2lnxlimx0+x2lnx

363.

limxx1/xlimxx1/x

364.

limx0x2/xlimx0x2/x

365.

limx0x21/xlimx0x21/x

366.

limxexxlimxexx

For the following exercises, evaluate the limits with either L’Hôpital’s rule or previously learned methods.

367.

limx3x29x3limx3x29x3

368.

limx3x29x+3limx3x29x+3

369.

limx0(1+x)−21xlimx0(1+x)−21x

370.

limxπ/2cosxπ2xlimxπ/2cosxπ2x

371.

limxπxπsinxlimxπxπsinx

372.

limx1x1sinxlimx1x1sinx

373.

limx0(1+x)n1xlimx0(1+x)n1x

374.

limx0(1+x)n1nxx2limx0(1+x)n1nxx2

375.

limx0sinxtanxx3limx0sinxtanxx3

376.

limx01+x1xxlimx01+x1xx

377.

limx0exx1x2limx0exx1x2

378.

limx0tanxxlimx0tanxx

379.

limx1x1lnxlimx1x1lnx

380.

limx0(x+1)1/xlimx0(x+1)1/x

381.

limx1xx3x1limx1xx3x1

382.

limx0+x2xlimx0+x2x

383.

limxxsin(1x)limxxsin(1x)

384.

limx0sinxxx2limx0sinxxx2

385.

limx0+xln(x4)limx0+xln(x4)

386.

limx(xex)limx(xex)

387.

limxx2exlimxx2ex

388.

limx03x2xxlimx03x2xx

389.

limx01+1/x11/xlimx01+1/x11/x

390.

limxπ/4(1tanx)cotxlimxπ/4(1tanx)cotx

391.

limxxe1/xlimxxe1/x

392.

limx0x1/cosxlimx0x1/cosx

393.

limx0+x1/xlimx0+x1/x

394.

limx0(11x)xlimx0(11x)x

395.

limx(11x)xlimx(11x)x

For the following exercises, use a calculator to graph the function and estimate the value of the limit, then use L’Hôpital’s rule to find the limit directly.

396.

[T] limx0ex1xlimx0ex1x

397.

[T] limx0xsin(1x)limx0xsin(1x)

398.

[T] limx1x11cos(πx)limx1x11cos(πx)

399.

[T] limx1e(x1)1x1limx1e(x1)1x1

400.

[T] limx1(x1)2lnxlimx1(x1)2lnx

401.

[T] limxπ1+cosxsinxlimxπ1+cosxsinx

402.

[T] limx0(cscx1x)limx0(cscx1x)

403.

[T] limx0+tan(xx)limx0+tan(xx)

404.

[T] limx0+lnxsinxlimx0+lnxsinx

405.

[T] limx0exexxlimx0exexx

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