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Math 213 - Partial Derivatives
Peter A. Perry
University of Kentucky
September 25, 2019
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 2: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/2.jpg)
Reminders
• Homework B1 on 14.1 is due on tonight!• You will have quiz #3 on 13.1-13.4 on Thursday• Homework B2 on 14.3 is due on Friday
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
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Unit II: Functions of Several Variables
13.3-4 Lecture 11: Velocity and Acceleration
14.1 Lecture 12: Functions of Several Variables14.3 Lecture 13: Partial Derivatives14.4 Lecture 14: Linear Approximation14.5 Lecture 15: Chain Rule, Implicit Differentiation14.6 Lecture 16: Directional Derivatives and the Gradient
14.7 Lecture 17: Maximum and Minimum Values, I14.7 Lecture 18: Maximum and Minimum Values, II14.8 Lecture 19: Lagrange Multipliers
15.1 Double Integrals15.2 Double Integrals over General Regions
Exam II Review
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 4: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/4.jpg)
Learning Goals
• Learn how to compute partial derivatives and know variousdifferent notations for them
• Understand the geometric interpretation of partial derivatives• Know how to compute higher partial derivatives• Understand their connection with partial differential equations
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 5: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/5.jpg)
Derivatives - One Variable
y
x
y = f (x)
a
(a, f (a))
The derivative of f at a is the limit
f ′(a) = limh→0
f (a + h)− f (a)h
if it exists.
f ′(a) is the slope of the tangent line tothe graph of f at the point (a, f (a)).
f ′(a) is also the instantaneous rate ofchange of y = f (x) at x = a
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 6: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/6.jpg)
Derivatives - One Variable
y
x
y = f (x)
a
(a, f (a))
(a + h, f (a + h))
(a, f (a))
The derivative of f at a is the limit
f ′(a) = limh→0
f (a + h)− f (a)h
if it exists.
f ′(a) is the slope of the tangent line tothe graph of f at the point (a, f (a)).
f ′(a) is also the instantaneous rate ofchange of y = f (x) at x = a
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 7: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/7.jpg)
Derivatives - One Variable
y
x
y = f (x)
a
(a, f (a))
The derivative of f at a is the limit
f ′(a) = limh→0
f (a + h)− f (a)h
if it exists.
f ′(a) is the slope of the tangent line tothe graph of f at the point (a, f (a)).
f ′(a) is also the instantaneous rate ofchange of y = f (x) at x = a
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 8: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/8.jpg)
Derivatives - One Variable
y
x
y = f (x)
a
(a, f (a))
The derivative of f at a is the limit
f ′(a) = limh→0
f (a + h)− f (a)h
if it exists.
f ′(a) is the slope of the tangent line tothe graph of f at the point (a, f (a)).
f ′(a) is also the instantaneous rate ofchange of y = f (x) at x = a
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 9: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/9.jpg)
Partial Derivatives - Two Variables
A function of two variables has two very natural rates of change:• The rate of change of z = f (x, y) with respect to x when y is fixed
• The rate of change of z = f (x, y) when respect to y when x is fixed
The first of these is called the partial derivative of f with respect to x, denoted ∂ f /∂x or fx
fx(a, b) = limh→0
f (a + h, b)− f (a, b)h
the second is called the partial derivative of f with respect to y, denoted ∂ f /∂y or fy
fy(a, b) = limh→0
f (a, b + h)− f (a, b)h
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 10: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/10.jpg)
Geometric Interpretation
−20
2 −2
0
20
20
xy
z
f (x, y) = x2(4− y2)
Given a function f (x, y) . . .
Compute fx(a, b) by setting y = b andvarying x:
fx(a, b) = limh→0
f (a + h, b)− f (a, b)h
Compute fy(a, b) by setting x = a andvarying y:
fx(a, b) = limh→0
f (a, b + h)− f (a, b)h
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 11: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/11.jpg)
Geometric Interpretation
−20
2 −2
0
20
20
xy
z
f (x, y) = x2(4− y2)
Given a function f (x, y) . . .
Compute fx(a, b) by setting y = b andvarying x:
fx(a, b) = limh→0
f (a + h, b)− f (a, b)h
Compute fy(a, b) by setting x = a andvarying y:
fx(a, b) = limh→0
f (a, b + h)− f (a, b)h
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 12: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/12.jpg)
Geometric Interpretation
−20
2 −2
0
20
20
xy
z
f (x, y) = x2(4− y2)
Given a function f (x, y) . . .
Compute fx(a, b) by setting y = b andvarying x:
fx(a, b) = limh→0
f (a + h, b)− f (a, b)h
Compute fy(a, b) by setting x = a andvarying y:
fx(a, b) = limh→0
f (a, b + h)− f (a, b)h
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 13: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/13.jpg)
Partial Derivatives
Rules for Finding Partial Derivatives of z = f (x, y)
1 To find fx , regard y as a constant and differentiate f (x, y) withrespect to x
2 To find fy, regard x as a constant and differentiate f (x, y) withrespect to y
Find both partial derivatives of the following functions:
1. f (x, y) = x4 + 5xy3 2. f (x, t) = t2e−x
3. g(u, v) = (u2 + v2)3 4. f (x, y) = sin(xy)
5. f (George, Fran) = (George)5 + (Fran)3
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
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Tangent Planes - Sneak Previewy
x
y = f (x)
a
(a, f (a))
In calculus of one variable, the derivativef ′(a) defines a tangent line to the graph off at (a, f (a)) by the equation
L(x) = f (a) + f ′(a)(x− a)
−2−1
01
2 −2
0
2
1
2
3
x
y
z
In calculus of two variables, the partialderivatives fx(a, b) and fy(a, b) define a tan-gent plane to the graph of f at (a, b, f (a, b))by
L(x, y) = f (a, b)
+ fx(a, b)(x− a) + fy(a, b)(y− b)
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 15: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/15.jpg)
More Partial Derivatives
Sometimes it’s useful to remember that, to compute a partial derivative like fx(x, 1),you can set y = 1 before you start computing.
Find the following partial derivatives.
1 fx(x, 1) if f (x, y) = xyyyy
sin(x)
2 fy(3, y) if f (x, y) = (x− 3) sin(cos(log(y)) + xy
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 16: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/16.jpg)
Higher Partials
We can compute higher-order partial derivatives just by repeating operations. We’llfind out what these partials actually mean later on!
Example Find the second partial derivatives of f (x, y) = x2y2
∂ f∂x
= fx(x, y) = 2xy2,∂ f∂y
= 2x2y
∂2 f∂x2 =
∂2 f∂x∂y
=
∂2 f∂y∂x
=∂2 f∂y2 =
Notations:∂2 f
∂y∂x= fxy = ( fx)y ,
∂2 f∂x∂y
= fyx =(
fy)
x
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
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Clairaut’s Theorem
Suppose f is defined on a disk D that contains the point (a, b). If the func-tions fxy and fyx are both continuous on D, then
fxy(a, b) = fyx(a, b)
Check Clairaut’s theorem for the function f (x, y) = x3y2 − sin(xy)
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 18: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/18.jpg)
Implicit Differentiation
You can find partial derivatives by implicit differentiation.
1 Find ∂z/∂x and ∂z/∂y if x2 + y2 + z2 = 1
2 Find ∂z/∂x and ∂z/∂y if ez = xyz
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 19: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/19.jpg)
Partial Differential Equations
Partial Differential Equations describe many physical phenomena. The unknownfunction is a function of two or more variables.
The wave equation for u(x, t), a function which, for each t gives a ‘snapshot’ of aone-dimensional traveling wave:
∂2u∂t2 =
1c2
∂2u∂x2
The heat equation for u(x, y, t), the temperature of a thin sheet at position (x, y) at time t:
∂u∂t
(x, y, t) = K(
∂2
∂x2 +∂2
∂y2
)u(x, y, t)
Laplace’s Equation for the electrostatic potential of a charge distribution ρ:(∂2
∂x2 +∂2
∂y2 +∂2
∂z2
)u(x, y, z) = 4πρ(x, y, z)
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 20: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/20.jpg)
The Wave Equation
∂2u∂t2 =
1c2
∂2u∂x2
02
46 0
2−1
0
1
x
t
u
u(x, t) = sin(x− 0.2 ∗ t) u(x, t) gives the height of a wave mov-ing down a channel as a function of dis-tance x and time t
For each fixed t, we get a “snapshot” ofthe wave
For each fixed x, we get the height ofthe wave, at that point, as a function oftime
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 21: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/21.jpg)
The Heat Equation
∂u∂t
(x, t) = K∂2
∂x2 u(x, t)
−2−1
01
2
1
2
0
2
x
t
u(x, t) = (4πt)−1/2e−x2/4t
For each t we get a “snapshot” of thedistribution of heat–at first heat con-centrates near x = 0, but then diffusesand cools as time moves forward
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives
![Page 22: Math 213 - Partial Derivativesperry/213-f19-perry/_assets/... · Partial Derivatives Rules for Finding Partial Derivatives of z = f(x,y) 1 To find fx, regard yas a constant and differentiate](https://reader034.vdocument.in/reader034/viewer/2022042612/5f6ff7618095716e045b5a60/html5/thumbnails/22.jpg)
Summary
• We learned how to compute partial derviatives of functions of several variablesand the various notations for them
• We interpreted partial derivatives geometrically
• We learned how to compute higher derivatives
• We saw how partial derivatives arise in equations that describe heat flow andwave propagation
Peter A. Perry University of Kentucky
Math 213 - Partial Derivatives