This document discusses indeterminate forms and L'Hospital's rule. It begins by introducing indeterminate forms as limits that are not obvious or defined, such as limits of the form 0/0 or infinity/infinity. It then presents L'Hospital's rule, which states that if the limit of a quotient of functions f(x) and g(x) is an indeterminate form, it is equal to the limit of the quotient of their derivatives, provided certain conditions are met. Several examples are worked through to demonstrate how to apply L'Hospital's rule to evaluate indeterminate limits. The document also discusses indeterminate products, which can be converted to indeterminate quotients before applying L'Hospital

1. APPLICATIONS OF DIFFERENTIATION
4

2. 4.4
Indeterminate Forms
and L’Hospital’s Rule
APPLICATIONS OF DIFFERENTIATION
In this section, we will learn:
How to evaluate functions whose
values cannot be found at certain points.

3. Suppose we are trying to analyze
the behavior of the function
Although F is not defined when x = 1,
we need to know how F behaves near 1.
ln
( )
1
x
F x
x


INDETERMINATE FORMS

4. In particular, we would like to know
the value of the limit
1
ln
lim
1
x
x
x
 
Expression 1
INDETERMINATE FORMS

5. In computing this limit, we can’t apply
Law 5 of limits (Section 2.3) because
the limit of the denominator is 0.
 In fact, although the limit in Expression 1 exists,
its value is not obvious because both numerator
and denominator approach 0 and is not defined.
0
0
INDETERMINATE FORMS

6. INDETERMINATE FORM —TYPE 0/0
In general, if we have a limit of the form
where both f(x) → 0 and g(x) → 0 as x → a,
then this limit may or may not exist.
It is called an indeterminate form of type .
 We met some limits of this type in Chapter 2.
( )
lim
( )
x a
f x
g x

0
0

7. INDETERMINATE FORMS
For rational functions, we can cancel
common factors:
2
2
1 1
1
( 1)
lim lim
( 1)( 1)
1
1
lim
1 2
x x
x
x x x x
x x
x
x
x
 

 

 

 


8. INDETERMINATE FORMS
We used a geometric argument
to show that:
0
sin
lim 1
x
x
x



9. INDETERMINATE FORMS
However, these methods do not work
for limits such as Expression 1.
 Hence, in this section, we introduce
a systematic method, known as l’Hospital’s Rule,
for the evaluation of indeterminate forms.

10. INDETERMINATE FORMS
Another situation in which a limit is
not obvious occurs when we look for
a horizontal asymptote of F and need
to evaluate the limit ln
lim
1
x
x
x
 
Expression 2

11. INDETERMINATE FORMS
It isn’t obvious how to evaluate this limit
because both numerator and denominator
become large as x → ∞.
There is a struggle between the two.
 If the numerator wins, the limit will be ∞.
 If the denominator wins, the answer will be 0.
 Alternatively, there may be some compromise—
the answer may be some finite positive number.

12. INDETERMINATE FORM —TYPE ∞/∞
In general, if we have a limit of the form
where both f(x) → ∞ (or -∞) and g(x) → ∞
(or -∞), then the limit may or may not exist.
It is called an indeterminate form of type ∞/∞.
( )
lim
( )
x a
f x
g x


13. INDETERMINATE FORMS
We saw in Section 2.6 that this type of limit
can be evaluated for certain functions—
including rational functions—by dividing the
numerator and denominator by the highest
power of x that occurs in the denominator.
 For instance,
2 2
2
2
1
1
1 1 0 1
lim lim
1 2 0 2
2 1
2
x x
x x
x
x
 

 
  




14. INDETERMINATE FORMS
This method, though, does not work
for limits such as Expression 2.
 However, L’Hospital’s Rule also applies to
this type of indeterminate form.

15. L’HOSPITAL’S RULE
Suppose f and g are differentiable and
g’(x) ≠ 0 on an open interval I that contains a
(except possibly at a).
Suppose
or that
 In other words, we have an indeterminate form
of type or ∞/∞.
lim ( ) 0 and lim ( ) 0
x a x a
f x g x
 
 
0
0
lim ( ) and lim ( )
x a x a
f x g x
 
   

16. L’HOSPITAL’S RULE
Then,
if the limit on the right exists
(or is ∞ or - ∞).
( ) '( )
lim lim
( ) '( )
x a x a
f x f x
g x g x
 


17. NOTE 1
L’Hospital’s Rule says that the limit of
a quotient of functions is equal to the limit of
the quotient of their derivatives—provided that
the given conditions are satisfied.
 It is especially important to verify the conditions
regarding the limits of f and g before using the rule.

18. NOTE 2
The rule is also valid for one-sided limits
and for limits at infinity or negative infinity.
 That is, ‘x → a’ can be replaced by any of
the symbols x → a+, x → a-, x → ∞, or x → - ∞.

19. NOTE 3
For the special case in which
f(a) = g(a) = 0, f’ and g’ are continuous,
and g’(a) ≠ 0, it is easy to see why
the rule is true.

20. NOTE 3
In fact, using the alternative form of
the definition of a derivative, we have:
( ) ( ) ( ) ( )
lim
'( ) '( )
lim lim
( ) ( ) ( ) ( )
'( ) '( )
lim
( ) ( )
lim
( ) ( )
( )
lim
( )
x a
x a x a
x a
x a
x a
f x f a f x f a
f x f a x a x a
g x g a g x g a
g x g a
x a x a
f x f a
g x g a
f x
g x

 



 
 
  
 
 





21. NOTE 3
It is more difficult to prove
the general version of l’Hospital’s
Rule.

22. L’HOSPITAL’S RULE
Find
 and
 Thus, we can apply l’Hospital’s Rule:
1
ln
lim
1
x
x
x
 
1 1
limln ln1 0 lim( 1) 0
x x
x x
 
   
1 1 1 1
(ln )
ln 1/ 1
lim lim lim lim 1
1 1
( 1)
x x x x
d
x
x x
dx
d
x x
x
dx
   
   


Example 1

23. L’HOSPITAL’S RULE
Calculate
 We have and
 So, l’Hospital’s Rule gives:
2
lim
x
x
e
x

2
lim lim
x
x x
e x
 
   
2
2
( )
lim lim lim
2
( )
x
x x
x x x
d
e
e e
dx
d
x x
x
dx
  
 
Example 2

24. L’HOSPITAL’S RULE
As ex → ∞ and 2x → ∞ as x → ∞, the limit
on the right side is also indeterminate.
However, a second application of l’Hospital’s
Rule gives:
2
lim lim lim
2 2
x x x
x x x
e e e
x x
  
   
Example 2

25. L’HOSPITAL’S RULE
Calculate
 As ln x → ∞ and as x → ∞,
l’Hospital’s Rule applies:
 Notice that the limit on the right side
is now indeterminate of type .
3
ln
lim
x
x
x

3
x  
2/3
1
3
3
ln 1/
lim lim
x x
x x
x
x

 

0
0
Example 3

26. L’HOSPITAL’S RULE
 However, instead of applying the rule
a second time as we did in Example 2,
we simplify the expression and see that
a second application is unnecessary:
2 / 3
1
3 3
3
ln 1/ 3
lim lim lim 0
x x x
x x
x
x x

  
  
Example 3

27. L’HOSPITAL’S RULE
Find
 Noting that both tan x – x → 0 and x3 → 0
as x → 0, we use l’Hospital’s Rule:
3
0
tan
lim
x
x x
x


2
3 2
0 0
tan sec 1
lim lim
3
x x
x x x
x x
 
 

Example 4

28. L’HOSPITAL’S RULE
 As the limit on the right side is still
indeterminate of type , we apply the rule
again:
Example 4
2 2
2
0 0
sec 1 2sec tan
lim lim
6
3
x x
x x x
x
x
 


0
0

29. L’HOSPITAL’S RULE
 Since , we simplify the
calculation by writing:
2
0
limsec 1
x
x


2
2
0 0 0
0
2sec tan 1 tan
lim limsec lim
6 3
1 tan
lim
3
x x x
x
x x x
x
x x
x
x
  



Example 4

30. L’HOSPITAL’S RULE
 We can evaluate this last limit either by
using l’Hospital’s Rule a third time or by
writing tan x as (sin x)/(cos x) and making
use of our knowledge of trigonometric limits.
Example 4

31. L’HOSPITAL’S RULE
Putting together all the steps,
we get:
2
3 2
0 0
2
0
2
0 0
tan sec 1
lim lim
3
2sec tan
lim
6
1 tan 1 sec 1
lim lim
3 3 1 3
x x
x
x x
x x x
x x
x x
x
x x
x
 

 
 


  
Example 4

32. L’HOSPITAL’S RULE
Find
 If we blindly attempted to use l-Hospital’s rule,
we would get:
sin
lim
1 cos
x
x
x

 
sin cos
lim lim
1 cos sin
x x
x x
x x
 
 
 
  

Example 5

33. This is wrong.
 Although the numerator sin x → 0 as x → π -,
notice that the denominator (1 - cos x) does not
approach 0.
 So, the rule can’t be applied here.
L’HOSPITAL’S RULE Example 5

34. L’HOSPITAL’S RULE
The required limit is, in fact, easy to find
because the function is continuous at π
and the denominator is nonzero there:
sin sin 0
lim 0
1 cos 1 cos 1 ( 1)
x
x
x





  
   
Example 5

35. L’HOSPITAL’S RULE
The example shows what can go wrong
if you use the rule without thinking.
 Other limits can be found using the rule, but
are more easily found by other methods.
 See Examples 3 and 5 in Section 2.3,
Example 3 in Section 2.6, and the discussion
at the beginning of the section.

36. L’HOSPITAL’S RULE
So, when evaluating any limit,
you should consider other methods
before using l’Hospital’s Rule.

37. INDETERMINATE PRODUCTS
If and (or -∞),
then it isn’t clear what the value
of , if any, will be.
lim ( ) 0
x a
f x

 lim ( )

 
x a
g x
lim ( ) ( )
x a
f x g x


38. INDETERMINATE PRODUCTS
There is a struggle between f and g.
 If f wins, the answer will be 0.
 If g wins, the answer will be ∞ (or -∞).
 Alternatively, there may be a compromise
where the answer is a finite nonzero number.

39. This kind of limit is called an indeterminate
form of type 0 . ∞.
 We can deal with it by writing the product fg
as a quotient:
 This converts the given limit into an indeterminate form
of type or ∞/∞, so that we can use l’Hospital’s Rule.
or
1/ 1/
f g
fg fg
g f
 
0
0
INDETERMINATE FORM—TYPE 0 . ∞

40. INDETERMINATE PRODUCTS
Evaluate
 The given limit is indeterminate because,
as x → 0+, the first factor (x) approaches 0,
whereas the second factor (ln x) approaches -∞.
0
lim ln
x
x x


Example 6

41. INDETERMINATE PRODUCTS
 Writing x = 1/(1/x), we have 1/x → ∞
as x → 0+.
 So, l’Hospital’s Rule gives:
Example 6
2
0 0 0
0
ln 1/
lim ln lim lim
1/ 1/
lim( ) 0
x x x
x
x x
x x
x x
x
  

  

 

  

42. In solving the example, another possible
option would have been to write:
 This gives an indeterminate form of the type 0/0.
 However, if we apply l’Hospital’s Rule, we get a more
complicated expression than the one we started with.
0 0
lim ln lim
1/ ln
x x
x
x x
x
 
 

INDETERMINATE PRODUCTS Note

43. INDETERMINATE PRODUCTS
In general, when we rewrite an
indeterminate product, we try to choose
the option that leads to the simpler limit.
Note

44. If and , then
the limit
is called an indeterminate form
of type ∞ - ∞.
lim ( )
x a
f x

  lim ( )
x a
g x

 
lim[ ( ) ( )]
x a
f x g x


INDETERMINATE FORM—TYPE ∞ - ∞

45. INDETERMINATE DIFFERENCES
Again, there is a contest between f and g.
 Will the answer be ∞ (f wins)?
 Will it be - ∞ (g wins)?
 Will they compromise on a finite number?

46. INDETERMINATE DIFFERENCES
To find out, we try to convert the difference
into a quotient (for instance, by using a
common denominator, rationalization, or
factoring out a common factor) so that
we have an indeterminate form of type
or ∞/∞.
0
0

47. INDETERMINATE DIFFERENCES
Compute
 First, notice that sec x → ∞ and tan x → ∞
as x → (π/2)
-
.
 So, the limit is indeterminate.
( / 2)
lim (sec tan )
x
x x
 


Example 7

48. INDETERMINATE DIFFERENCES
Here, we use a common denominator:
 Note that the use of l’Hospital’s Rule is justified
because 1 – sin x → 0 and cos x → 0 as x → (π/2)
-
.
( /2) ( /2)
( /2)
( /2)
1 sin
lim (sec tan ) lim
cos cos
1 sin
lim
cos
cos
lim 0
sin
x x
x
x
x
x x
x x
x
x
x
x
 


 


 


 
  
 
 



 

Example 7

49. INDETERMINATE POWERS
Several indeterminate forms arise from
the limit ( )
lim[ ( )]g x
x a
f x

0
0
1. lim ( ) 0 and lim ( ) 0 type0
2. lim ( ) and lim ( ) 0 type
3. lim ( ) 1 and lim ( ) type1
x a x a
x a x a
x a x a
f x g x
f x g x
f x g x
 
 

 
 
   
  

50. INDETERMINATE POWERS
Each of these three cases can be treated
in either of two ways.
 Taking the natural logarithm:
 Writing the function as an exponential:
( )
Let [ ( )] , then ln ( )ln ( )
g x
y f x y g x f x
 
( ) ( )ln ( )
[ ( )]g x g x f x
f x e


51. INDETERMINATE POWERS
Recall that both these methods were used
in differentiating such functions.
 In either method, we are led to the indeterminate
product g(x) ln f(x), which is of type 0 . ∞.

52. INDETERMINATE POWERS
Calculate
 First, notice that, as x → 0+, we have
1 + sin 4x → 1 and cot x → ∞.
 So, the given limit is indeterminate.
cot
0
lim(1 sin 4 ) x
x
x



Example 8

53. INDETERMINATE POWERS
Let y = (1 + sin 4x)cot x
Then, ln y = ln[(1 + sin 4x)cot x]
= cot x ln(1 + sin 4x)
Example 8

54. INDETERMINATE POWERS
So, l’Hospital’s Rule gives:
0 0
2
0
ln(1 sin 4 )
lim ln lim
tan
4cos4
1 sin 4
lim 4
sec
x x
x
x
y
x
x
x
x
 

 




 
Example 8

55. INDETERMINATE POWERS
So far, we have computed the limit of ln y.
However, what we want is the limit of y.
 To find this, we use the fact that y = eln y:
cot
0 0
ln 4
0
lim(1 sin 4 ) lim
lim
x
x x
y
x
x y
e e
 

 

 
 
Example 8

56. INDETERMINATE POWERS
Find
 Notice that this limit is indeterminate
since 0
x
= 0 for any x > 0 but x0 = 1
for any x ≠ 0.
0
lim x
x
x


Example 9

57. INDETERMINATE POWERS
We could proceed as in Example 8 or by
writing the function as an exponential:
xx = (eln x)x = ex ln x
 In Example 6, we used l’Hospital’s Rule
to show that
 Therefore,
0
lim ln 0
x
x x



ln 0
0 0
lim lim 1
x x x
x x
x e e
 
 
  
Example 9