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Recognizing indeterminate forms and resolving them with L'Hôpital's Rule
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Section 4.5Indeterminate Forms and L’Hopital’s Rule
Math 1a
November 26, 2007
Announcements
I Special review session on optimization problems:Tues 11/27 (tomorrow) 7:00–9:00 (SC 507)
I my next office hours: today 1–2, tomorrow 3–4 (SC 323)
I MT II Review session: Sunday, 11/2, 7:30–9:00 (SC Hall D)
I Midterm II: Tues 12/4 7:00-9:00pm (SC Hall B)
Outline
Indeterminate Forms
L’Hopital’s RuleApplication to Indeterminate ProductsApplication to Indeterminate DifferencesApplication to Indeterminate PowersSummary
The Cauchy Mean Value Theorem (Bonus)
Recall
Recall the limit laws from Chapter 2.
I Limit of a sum is the sum of the limits
I Limit of a difference is the difference of the limits
I Limit of a product is the product of the limits
I Limit of a quotient is the quotient of the limits ... whoops!This is true as long as you don’t try to divide by zero.
Recall
Recall the limit laws from Chapter 2.
I Limit of a sum is the sum of the limits
I Limit of a difference is the difference of the limits
I Limit of a product is the product of the limits
I Limit of a quotient is the quotient of the limits ... whoops!This is true as long as you don’t try to divide by zero.
Recall
Recall the limit laws from Chapter 2.
I Limit of a sum is the sum of the limits
I Limit of a difference is the difference of the limits
I Limit of a product is the product of the limits
I Limit of a quotient is the quotient of the limits ... whoops!This is true as long as you don’t try to divide by zero.
Recall
Recall the limit laws from Chapter 2.
I Limit of a sum is the sum of the limits
I Limit of a difference is the difference of the limits
I Limit of a product is the product of the limits
I Limit of a quotient is the quotient of the limits ... whoops!This is true as long as you don’t try to divide by zero.
We know dividing by zero is bad. Most of the time, if you have anumerator which approaches a finite number and a denominatorwhich approaches zero, the quotient approaches some kind ofinfinity. An exception would be something like
limx→∞
11x sin x
= limx→∞
x sec x .
which doesn’t exist.
Even worse is the situation where the numerator and denominatorboth go to zero.
We know dividing by zero is bad. Most of the time, if you have anumerator which approaches a finite number and a denominatorwhich approaches zero, the quotient approaches some kind ofinfinity. An exception would be something like
limx→∞
11x sin x
= limx→∞
x sec x .
which doesn’t exist.Even worse is the situation where the numerator and denominatorboth go to zero.
Experiments
I limx→0+
sin2 x
x
= 0
I limx→0
x
sin2 x
does not exist
I limx→0
sin2 x
sin x2
= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 x
does not exist
I limx→0
sin2 x
sin x2
= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 x
does not exist
I limx→0
sin2 x
sin x2
= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2
= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2
= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2= 1
I limx→0
sin 3x
sin x
= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2= 1
I limx→0
sin 3x
sin x= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Experiments
I limx→0+
sin2 x
x= 0
I limx→0
x
sin2 xdoes not exist
I limx→0
sin2 x
sin x2= 1
I limx→0
sin 3x
sin x= 3
All of these are of the form0
0, and since we can get different
answers in different cases, we say this form is indeterminate.
Language NoteIt depends on what the meaning of the word “is” is
I Be careful with the language here. We are not saying that the
limit in each case “is”0
0, and therefore nonexistent because
this expression is undefined.
I The limit is of the form0
0, which means we cannot evaluate it
with our limit laws.
Indeterminate forms are like Tug Of War
Which side wins depends on which side is stronger.
Outline
Indeterminate Forms
L’Hopital’s RuleApplication to Indeterminate ProductsApplication to Indeterminate DifferencesApplication to Indeterminate PowersSummary
The Cauchy Mean Value Theorem (Bonus)
QuestionIf f and g are lines and f (a) = g(a) = 0, what is
limx→a
f (x)
g(x)?
SolutionThe functions f and g can be written in the form
f (x) = m1(x − a)
g(x) = m2(x − a)
Sof (x)
g(x)=
m1
m2=
f ′(x)
g ′(x).
But what if the functions aren’t linear? If only there were a way todeal with functions which were only approximately linear!
QuestionIf f and g are lines and f (a) = g(a) = 0, what is
limx→a
f (x)
g(x)?
SolutionThe functions f and g can be written in the form
f (x) = m1(x − a)
g(x) = m2(x − a)
Sof (x)
g(x)=
m1
m2=
f ′(x)
g ′(x).
But what if the functions aren’t linear? If only there were a way todeal with functions which were only approximately linear!
QuestionIf f and g are lines and f (a) = g(a) = 0, what is
limx→a
f (x)
g(x)?
SolutionThe functions f and g can be written in the form
f (x) = m1(x − a)
g(x) = m2(x − a)
Sof (x)
g(x)=
m1
m2=
f ′(x)
g ′(x).
But what if the functions aren’t linear? If only there were a way todeal with functions which were only approximately linear!
Theorem (L’Hopital’s Rule)
Suppose f and g are differentiable functions and g ′(x) 6= 0 near a(except possibly at a). Suppose that
limx→a
f (x) = 0 and limx→a
g(x) = 0
or
limx→a
f (x) = ±∞ and limx→a
g(x) = ±∞
Then
limx→a
f (x)
g(x)= lim
x→a
f ′(x)
g ′(x),
if the limit on the right-hand side is finite, ∞, or −∞.
L’Hopital’s rule also applies for limits of the form∞∞
.
Theorem (L’Hopital’s Rule)
Suppose f and g are differentiable functions and g ′(x) 6= 0 near a(except possibly at a). Suppose that
limx→a
f (x) = 0 and limx→a
g(x) = 0
or
limx→a
f (x) = ±∞ and limx→a
g(x) = ±∞
Then
limx→a
f (x)
g(x)= lim
x→a
f ′(x)
g ′(x),
if the limit on the right-hand side is finite, ∞, or −∞.
L’Hopital’s rule also applies for limits of the form∞∞
.
Meet the Mathematician
I wanted to be a militaryman, but poor eyesightforced him into math
I did some math on hisown (solved the“brachistocroneproblem”)
I paid a stipend to JohannBernoulli, who provedthis theorem and namedit after him! Guillaume Franois Antoine,
Marquis de L’Hopital(1661–1704)
How does this affect our examples above?
Example
limx→0
sin2 x
x
H= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)
H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)
= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin x
H= lim
x→0
3 cos 3x
cos x= 3.
How does this affect our examples above?
Example
limx→0
sin2 x
xH= lim
x→0
2 sin x cos x
1= 0.
Example
limx→0
sin2 x
sin x2
H= lim
x→0
�2 sin x cos x
(cos x2) (�2x)H= lim
x→0
cos2 x − sin2 x
cos x2 − x2 sin(x2)= 1
Example
limx→0
sin 3x
sin xH= lim
x→0
3 cos 3x
cos x= 3.
Sketch of Proof of L’Hopital’s Rule
Let x be a number close to a. We know thatf (x)− f (a)
x − a= f ′(c),
for some c ∈ (a, x); alsog(x)− g(a)
x − a= g ′(d), for some
d ∈ (a, x). This means
f (x)
g(x)≈ f ′(c)
g ′(d).
The miracle of the MVT is that a tweaking of it allows us toassume c = d , so that
f (x)
g(x)=
f ′(c)
g ′(c).
The number c depends on x and since it is between a and x , wemust have
limx→a
c(x) = a.
Beware of Red Herrings
Example
Findlimx→0
x
cos x
SolutionThe limit of the denominator is 1, not 0, so L’Hopital’s rule doesnot apply. The limit is 0.
Beware of Red Herrings
Example
Findlimx→0
x
cos x
SolutionThe limit of the denominator is 1, not 0, so L’Hopital’s rule doesnot apply. The limit is 0.
TheoremLet r be any positive number. Then
limx→∞
ex
x r=∞.
Proof.If r is a positive integer, then apply L’Hopital’s rule r times to thefraction. You get
limx→∞
ex
x r
H= . . .
H= lim
x→∞
ex
r !=∞.
If r is not an integer, let n = [[x ]] and m = n + 1. Then if x > 1,xn < x r < xm, so
ex
xn>
ex
x r>
ex
xm.
Now apply the Squeeze Theorem.
TheoremLet r be any positive number. Then
limx→∞
ex
x r=∞.
Proof.If r is a positive integer, then apply L’Hopital’s rule r times to thefraction. You get
limx→∞
ex
x r
H= . . .
H= lim
x→∞
ex
r !=∞.
If r is not an integer, let n = [[x ]] and m = n + 1. Then if x > 1,xn < x r < xm, so
ex
xn>
ex
x r>
ex
xm.
Now apply the Squeeze Theorem.
TheoremLet r be any positive number. Then
limx→∞
ex
x r=∞.
Proof.If r is a positive integer, then apply L’Hopital’s rule r times to thefraction. You get
limx→∞
ex
x r
H= . . .
H= lim
x→∞
ex
r !=∞.
If r is not an integer, let n = [[x ]] and m = n + 1. Then if x > 1,xn < x r < xm, so
ex
xn>
ex
x r>
ex
xm.
Now apply the Squeeze Theorem.
Indeterminate products
Example
Findlim
x→0+
√x ln x
SolutionJury-rig the expression to make an indeterminate quotient. Thenapply L’Hopital’s Rule:
limx→0+
√x ln x = lim
x→0+
ln x1/√
x
H= lim
x→0+
x−1
−12x−3/2
= limx→0+
−2√
x = 0
Indeterminate products
Example
Findlim
x→0+
√x ln x
SolutionJury-rig the expression to make an indeterminate quotient. Thenapply L’Hopital’s Rule:
limx→0+
√x ln x = lim
x→0+
ln x1/√
x
H= lim
x→0+
x−1
−12x−3/2
= limx→0+
−2√
x = 0
Indeterminate differences
Example
limx→0
(1
x− cot 2x
)
This limit is of the form ∞−∞, which is indeterminate.
SolutionAgain, rig it to make an indeterminate quotient.
limx→0+
1− x cot 2x
xH= lim
x→0+
2x csc2(2x)− cot(2x)
1= lim
x→0+
2x − cos 2x
sin2 x sin 2xH= lim
x→0+
2 + 2 sin 2x
2 cos 2x sin2 x + 2 cos x sin x sin 2x
=∞
Indeterminate differences
Example
limx→0
(1
x− cot 2x
)This limit is of the form ∞−∞, which is indeterminate.
SolutionAgain, rig it to make an indeterminate quotient.
limx→0+
1− x cot 2x
xH= lim
x→0+
2x csc2(2x)− cot(2x)
1= lim
x→0+
2x − cos 2x
sin2 x sin 2xH= lim
x→0+
2 + 2 sin 2x
2 cos 2x sin2 x + 2 cos x sin x sin 2x
=∞
Indeterminate differences
Example
limx→0
(1
x− cot 2x
)This limit is of the form ∞−∞, which is indeterminate.
SolutionAgain, rig it to make an indeterminate quotient.
limx→0+
1− x cot 2x
xH= lim
x→0+
2x csc2(2x)− cot(2x)
1= lim
x→0+
2x − cos 2x
sin2 x sin 2xH= lim
x→0+
2 + 2 sin 2x
2 cos 2x sin2 x + 2 cos x sin x sin 2x
=∞
Indeterminate powers
Example
limx→0+
(1− 2x)1/x
Take the logarithm:
ln(
limx→0
(1− 2x)1/x)
= limx→0
ln(
(1− 2x)1/x)
= limx→0
1
xln(1− 2x)
This limit is of the form0
0, so we can use L’Hopital:
limx→0
1
xln(1− 2x)
H= lim
x→0
−21−2x
1= −2
This is not the answer, it’s the log of the answer! So the answerwe want is e−2.
Indeterminate powers
Example
limx→0+
(1− 2x)1/x
Take the logarithm:
ln(
limx→0
(1− 2x)1/x)
= limx→0
ln(
(1− 2x)1/x)
= limx→0
1
xln(1− 2x)
This limit is of the form0
0, so we can use L’Hopital:
limx→0
1
xln(1− 2x)
H= lim
x→0
−21−2x
1= −2
This is not the answer, it’s the log of the answer! So the answerwe want is e−2.
Example
limx→0
(3x)4x
Solution
ln limx→0+
(3x)4x = limx→0+
ln(3x)4x = limx→0+
4x ln(3x)
= limx→0+
ln(3x)1/4x
H= lim
x→0+
3/3x
−1/4x2
= limx→0+
(−4x) = 0
So the answer is e0 = 1.
Example
limx→0
(3x)4x
Solution
ln limx→0+
(3x)4x = limx→0+
ln(3x)4x = limx→0+
4x ln(3x)
= limx→0+
ln(3x)1/4x
H= lim
x→0+
3/3x
−1/4x2
= limx→0+
(−4x) = 0
So the answer is e0 = 1.
Summary
Form Method
00 L’Hopital’s rule directly
∞∞ L’Hopital’s rule directly
0 · ∞ jiggle to make 00 or ∞∞ .
∞−∞ factor to make an indeterminate product
00 take ln to make an indeterminate product
∞0 ditto
1∞ ditto
Outline
Indeterminate Forms
L’Hopital’s RuleApplication to Indeterminate ProductsApplication to Indeterminate DifferencesApplication to Indeterminate PowersSummary
The Cauchy Mean Value Theorem (Bonus)
The Cauchy Mean Value Theorem (Bonus)
Apply the MVT to the function
h(x) = (f (b)− f (a))g(x)− (g(b)− g(a))f (x).
We have h(a) = h(b). So there exists a c in (a, b) such thath′(c) = 0. Thus
(f (b)− f (a))g ′(c) = (g(b)− g(a))f ′(c)
This is how L’Hopitalis proved.