lesson 10: the chain rule (slides)
DESCRIPTION
The derivative of a composition of functions is the product of the derivatives of those functions. This rule is important because compositions are so powerful.TRANSCRIPT
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Sec on 2.5The Chain Rule
V63.0121.011: Calculus IProfessor Ma hew Leingang
New York University
February 23, 2011
Announcements
I Quiz 2 next week on§§1.5, 1.6, 2.1, 2.2
I Midterm March 7 on allsec ons in class (coversall sec ons up to 2.5)
ObjectivesI Given a compoundexpression, write it as acomposi on of func ons.
I Understand and applythe Chain Rule for thederiva ve of acomposi on of func ons.
I Understand and useNewtonian and Leibniziannota ons for the ChainRule.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
.
.g . f.x .g(x). f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g
. f
.x .g(x)
. f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g . f.x .g(x)
. f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g . f.x .g(x). f(g(x))
.f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g . f.x .g(x). f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g . f.x .g(x). f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
Outline
Heuris csAnalogyThe Linear Case
The chain rule
Examples
AnalogyThink about riding a bike. Togo faster you can either:
I pedal fasterI change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R..
.
radius of front sprocket
θ
r..
.
radius of back sprocket
And so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
AnalogyThink about riding a bike. Togo faster you can either:
I pedal faster
I change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R..
.
radius of front sprocket
θ
r..
.
radius of back sprocket
And so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
AnalogyThink about riding a bike. Togo faster you can either:
I pedal fasterI change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R..
.
radius of front sprocket
θ
r..
.
radius of back sprocket
And so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
AnalogyThink about riding a bike. Togo faster you can either:
I pedal fasterI change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R..
.
radius of front sprocket
θ
r..
.
radius of back sprocketAnd so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
AnalogyThink about riding a bike. Togo faster you can either:
I pedal fasterI change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R...
radius of front sprocket
θ
r...
radius of back sprocket
And so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
AnalogyThink about riding a bike. Togo faster you can either:
I pedal fasterI change gears
..
Image credit: SpringSun
The angular posi on (φ) of the back wheel depends on the posi onof the front sprocket (θ):
φ(θ) =R..
.
radius of front sprocket
θ
r..
.
radius of back sprocket
And so the angular speed of the back wheel depends on thederiva ve of this func on and the speed of the front sprocket.
The Linear CaseQues onLet f(x) = mx+ b and g(x) = m′x+ b′. What can you say about the composi on?
Answer
I f(g(x)) = m(m′x+ b′) + b = (mm′)x+ (mb′ + b)
I The composi on is also linear
I The slope of the composi on is the product of the slopes of the twofunc ons.
The deriva ve is supposed to be a local lineariza on of a func on. So thereshould be an analog of this property in deriva ves.
The Linear CaseQues onLet f(x) = mx+ b and g(x) = m′x+ b′. What can you say about the composi on?
Answer
I f(g(x)) = m(m′x+ b′) + b = (mm′)x+ (mb′ + b)
I The composi on is also linear
I The slope of the composi on is the product of the slopes of the twofunc ons.
The deriva ve is supposed to be a local lineariza on of a func on. So thereshould be an analog of this property in deriva ves.
The Linear CaseQues onLet f(x) = mx+ b and g(x) = m′x+ b′. What can you say about the composi on?
Answer
I f(g(x)) = m(m′x+ b′) + b = (mm′)x+ (mb′ + b)
I The composi on is also linear
I The slope of the composi on is the product of the slopes of the twofunc ons.
The deriva ve is supposed to be a local lineariza on of a func on. So thereshould be an analog of this property in deriva ves.
The Linear CaseQues onLet f(x) = mx+ b and g(x) = m′x+ b′. What can you say about the composi on?
Answer
I f(g(x)) = m(m′x+ b′) + b = (mm′)x+ (mb′ + b)
I The composi on is also linear
I The slope of the composi on is the product of the slopes of the twofunc ons.
The deriva ve is supposed to be a local lineariza on of a func on. So thereshould be an analog of this property in deriva ves.
The Linear CaseQues onLet f(x) = mx+ b and g(x) = m′x+ b′. What can you say about the composi on?
Answer
I f(g(x)) = m(m′x+ b′) + b = (mm′)x+ (mb′ + b)
I The composi on is also linear
I The slope of the composi on is the product of the slopes of the twofunc ons.
The deriva ve is supposed to be a local lineariza on of a func on. So thereshould be an analog of this property in deriva ves.
The Nonlinear CaseLet u = g(x) and y = f(u). Suppose x is changed by a small amount∆x. Then
f′(u) ≈ ∆y∆u
=⇒ ∆y ≈ f′(u)∆u
andg′(x) ≈ ∆y
∆x=⇒ ∆u ≈ g′(x)∆x.
So∆y ≈ f′(u)g′(x)∆x =⇒ ∆y
∆x≈ f′(u)g′(x)
Outline
Heuris csAnalogyThe Linear Case
The chain rule
Examples
Theorem of the day: The chain ruleTheoremLet f and g be func ons, with g differen able at x and f differen ableat g(x). Then f ◦ g is differen able at x and
(f ◦ g)′(x) = f′(g(x))g′(x)
In Leibnizian nota on, let y = f(u) and u = g(x). Then
dydx
=dydu
dudx
..dy��du
��dudx
ObservationsI Succinctly, the deriva ve of acomposi on is the productof the deriva ves
I The only complica on iswhere these deriva ves areevaluated: at the same pointthe func ons are
I In Leibniz nota on, the ChainRule looks like cancella on of(fake) frac ons
..
Image credit: ooOJasonOoo
Theorem of the day: The chain ruleTheoremLet f and g be func ons, with g differen able at x and f differen ableat g(x). Then f ◦ g is differen able at x and
(f ◦ g)′(x) = f′(g(x))g′(x)
In Leibnizian nota on, let y = f(u) and u = g(x). Then
dydx
=dydu
dudx
..dy��du
��dudx
ObservationsI Succinctly, the deriva ve of acomposi on is the productof the deriva ves
I The only complica on iswhere these deriva ves areevaluated: at the same pointthe func ons are
I In Leibniz nota on, the ChainRule looks like cancella on of(fake) frac ons
..
Image credit: ooOJasonOoo
CompositionsSee Section 1.2 for review
Defini onIf f and g are func ons, the composi on (f ◦ g)(x) = f(g(x))means “do g first,then f.”
..g . f.x .g(x). f(g(x)).f ◦ g
Our goal for the day is to understand how the deriva ve of the composi on oftwo func ons depends on the deriva ves of the individual func ons.
ObservationsI Succinctly, the deriva ve of acomposi on is the productof the deriva ves
I The only complica on iswhere these deriva ves areevaluated: at the same pointthe func ons are
I In Leibniz nota on, the ChainRule looks like cancella on of(fake) frac ons
..
Image credit: ooOJasonOoo
Theorem of the day: The chain ruleTheoremLet f and g be func ons, with g differen able at x and f differen ableat g(x). Then f ◦ g is differen able at x and
(f ◦ g)′(x) = f′(g(x))g′(x)
In Leibnizian nota on, let y = f(u) and u = g(x). Then
dydx
=dydu
dudx
..dy��du
��dudx
Theorem of the day: The chain ruleTheoremLet f and g be func ons, with g differen able at x and f differen ableat g(x). Then f ◦ g is differen able at x and
(f ◦ g)′(x) = f′(g(x))g′(x)
In Leibnizian nota on, let y = f(u) and u = g(x). Then
dydx
=dydu
dudx
..dy��du
��dudx
Outline
Heuris csAnalogyThe Linear Case
The chain rule
Examples
ExampleExample
let h(x) =√3x2 + 1. Find h′(x).
Solu onFirst, write h as f ◦ g. Let f(u) =
√u and g(x) = 3x2 + 1. Then
f′(u) = 12u
−1/2, and g′(x) = 6x. So
h′(x) = 12u
−1/2(6x) = 12(3x
2 + 1)−1/2(6x) =3x√
3x2 + 1
ExampleExample
let h(x) =√3x2 + 1. Find h′(x).
Solu onFirst, write h as f ◦ g.
Let f(u) =√u and g(x) = 3x2 + 1. Then
f′(u) = 12u
−1/2, and g′(x) = 6x. So
h′(x) = 12u
−1/2(6x) = 12(3x
2 + 1)−1/2(6x) =3x√
3x2 + 1
ExampleExample
let h(x) =√3x2 + 1. Find h′(x).
Solu onFirst, write h as f ◦ g. Let f(u) =
√u and g(x) = 3x2 + 1.
Thenf′(u) = 1
2u−1/2, and g′(x) = 6x. So
h′(x) = 12u
−1/2(6x) = 12(3x
2 + 1)−1/2(6x) =3x√
3x2 + 1
ExampleExample
let h(x) =√3x2 + 1. Find h′(x).
Solu onFirst, write h as f ◦ g. Let f(u) =
√u and g(x) = 3x2 + 1. Then
f′(u) = 12u
−1/2, and g′(x) = 6x. So
h′(x) = 12u
−1/2(6x)
= 12(3x
2 + 1)−1/2(6x) =3x√
3x2 + 1
ExampleExample
let h(x) =√3x2 + 1. Find h′(x).
Solu onFirst, write h as f ◦ g. Let f(u) =
√u and g(x) = 3x2 + 1. Then
f′(u) = 12u
−1/2, and g′(x) = 6x. So
h′(x) = 12u
−1/2(6x) = 12(3x
2 + 1)−1/2(6x) =3x√
3x2 + 1
Corollary
Corollary (The Power Rule Combined with the Chain Rule)
If n is any real number and u = g(x) is differen able, then
ddx
(un) = nun−1dudx
.
Does order matter?Example
Findddx
(sin 4x) and compare it toddx
(4 sin x).
Solu on
I For the first, let u = 4x and y = sin(u). Then
dydx
=dydu
· dudx
= cos(u) · 4 = 4 cos 4x.
I For the second, let u = sin x and y = 4u. Then
dydx
=dydu
· dudx
= 4 · cos x
Does order matter?Example
Findddx
(sin 4x) and compare it toddx
(4 sin x).
Solu onI For the first, let u = 4x and y = sin(u). Then
dydx
=dydu
· dudx
= cos(u) · 4 = 4 cos 4x.
I For the second, let u = sin x and y = 4u. Then
dydx
=dydu
· dudx
= 4 · cos x
Does order matter?Example
Findddx
(sin 4x) and compare it toddx
(4 sin x).
Solu onI For the first, let u = 4x and y = sin(u). Then
dydx
=dydu
· dudx
= cos(u) · 4 = 4 cos 4x.
I For the second, let u = sin x and y = 4u. Then
dydx
=dydu
· dudx
= 4 · cos x
Order matters!Example
Findddx
(sin 4x) and compare it toddx
(4 sin x).
Solu onI For the first, let u = 4x and y = sin(u). Then
dydx
=dydu
· dudx
= cos(u) · 4 = 4 cos 4x.
I For the second, let u = sin x and y = 4u. Then
dydx
=dydu
· dudx
= 4 · cos x
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu on
ddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)= 2
(3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu onddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)
= 2(
3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu onddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)= 2
(3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu onddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)= 2
(3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu onddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)= 2
(3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
Example
Let f(x) =(
3√x5 − 2+ 8
)2. Find f′(x).
Solu onddx
(3√x5 − 2+ 8
)2= 2
(3√x5 − 2+ 8
) ddx
(3√x5 − 2+ 8
)= 2
(3√x5 − 2+ 8
) ddx
3√x5 − 2
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3 ddx
(x5 − 2)
= 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
=103x4(
3√x5 − 2+ 8
)(x5 − 2)−2/3
A metaphorThink about peeling an onion:
f(x) =(
3√
x5︸︷︷︸�5
−2︸ ︷︷ ︸3√�
+8
︸ ︷︷ ︸�+8
)2
︸ ︷︷ ︸�2
..
Image credit: photobunny
f′(x) = 2(
3√x5 − 2+ 8
)13(x
5 − 2)−2/3(5x4)
Combining techniquesExample
Findddx
((x3 + 1)10 sin(4x2 − 7)
)
Solu onThe “last” part of the func on is the product, so we apply the product rule. Eachfactor’s deriva ve requires the chain rule:
ddx
((x3 + 1)10 · sin(4x2 − 7)
)=
(ddx
(x3 + 1)10)· sin(4x2 − 7) + (x3 + 1)10 ·
(ddx
sin(4x2 − 7))
= 10(x3 + 1)9(3x2) sin(4x2 − 7) + (x3 + 1)10 · cos(4x2 − 7)(8x)
Combining techniquesExample
Findddx
((x3 + 1)10 sin(4x2 − 7)
)Solu onThe “last” part of the func on is the product, so we apply the product rule. Eachfactor’s deriva ve requires the chain rule:
ddx
((x3 + 1)10 · sin(4x2 − 7)
)=
(ddx
(x3 + 1)10)· sin(4x2 − 7) + (x3 + 1)10 ·
(ddx
sin(4x2 − 7))
= 10(x3 + 1)9(3x2) sin(4x2 − 7) + (x3 + 1)10 · cos(4x2 − 7)(8x)
Combining techniquesExample
Findddx
((x3 + 1)10 sin(4x2 − 7)
)Solu onThe “last” part of the func on is the product, so we apply the product rule. Eachfactor’s deriva ve requires the chain rule:
ddx
((x3 + 1)10 · sin(4x2 − 7)
)=
(ddx
(x3 + 1)10)· sin(4x2 − 7) + (x3 + 1)10 ·
(ddx
sin(4x2 − 7))
= 10(x3 + 1)9(3x2) sin(4x2 − 7) + (x3 + 1)10 · cos(4x2 − 7)(8x)
Combining techniquesExample
Findddx
((x3 + 1)10 sin(4x2 − 7)
)Solu onThe “last” part of the func on is the product, so we apply the product rule. Eachfactor’s deriva ve requires the chain rule:
ddx
((x3 + 1)10 · sin(4x2 − 7)
)=
(ddx
(x3 + 1)10)· sin(4x2 − 7) + (x3 + 1)10 ·
(ddx
sin(4x2 − 7))
= 10(x3 + 1)9(3x2) sin(4x2 − 7) + (x3 + 1)10 · cos(4x2 − 7)(8x)
Related rates of change in the oceanQues onThe area of a circle, A = πr2, changes as its radiuschanges. If the radius changes with respect to me,the change in area with respect to me is
A.dAdr
= 2πr
B.dAdt
= 2πr+drdt
C.dAdt
= 2πrdrdt
D. not enough informa on..
Image credit: Jim Frazier
Related rates of change in the oceanQues onThe area of a circle, A = πr2, changes as its radiuschanges. If the radius changes with respect to me,the change in area with respect to me is
A.dAdr
= 2πr
B.dAdt
= 2πr+drdt
C.dAdt
= 2πrdrdt
D. not enough informa on..
Image credit: Jim Frazier
Summary
I The deriva ve of acomposi on is theproduct of deriva ves
I In symbols:(f ◦ g)′(x) = f′(g(x))g′(x)
I Calculus is like an onion,and not because it makesyou cry!