2. Objectives :
• Identify domain and range of function of
two and three variables.
• Sketch graphs and level of curves of
functions of two and three variables
• Compute first and second partial
derivatives.
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Definition :
A function of two variables is a rule f that
assigns to each ordered pair (x,y)
in a set D a unique number z = f (x,y).
The set D is called the domain of the
function, and the corresponding values
of z = f (x,y) constitute the range of f .
4. Find the domains and range of the following functions
and evaluate f at the given points.
1
a) , ;
1
Eva1uate 3,2
6
: , 1 0, 1 , 3,2
2
, ,
b) , ;
2
Eva1uate 2,3 , 2,1
x y
f x y
x
f
Answer D x y x y x f
z f x y range is z z
xy
f x y
x y
f f
EXAMPLE 1
5. Find the domains and range of the following functions
and evaluate f at the given points.
2 2
2 2
2
c) , 25 ;
Eva1uate 2,3 , 7,4
: , 25 ,
, , 0 5
) , 3ln
Eva1uate 3,2
f x y x y
f f
Answer D x y x y
z f x y range is z z
d f x y y x
f
Continue…
6. A set of points where f is a constant is
called a level curve. A set of level
curves is called contour map.
20. rahimahj@ump.edu.my
Limits Along Curves
For a function one variable there two one-sided limits at a point
namely
reflecting the fact that there are only two directions from which x
can approach
0x
)(limand)(lim
00
xfxf
xxxx
0x
Function of
2 variables
Function of
3 variables
))(),((lim),(lim
0
)Calong(
00 ),(),(
tytxfyxf
ttyxyx
))(),(),((lim),,(lim
0
)Calong(
000 ),,(),,(
tztytxfzyxf
ttzyxzyx
21. EXAMPLE 3
Find the limit of along22
),(
yx
xy
yxf
axis-the)( xa
axis-the)( yb
xyc linethe)(
xyd linethe)(
2
parabolathe)( xye
22. The process of differentiating a function of
several variables with respect to one of its
variables while keeping the other variable(s)
fixed is called partial differentiation, and the
resulting derivative is a partial derivative of the
function.
23. The derivative of a function of a single variable
f is defined to be the limit of difference
quotient, namely,
Partial derivatives with respect to x or y are
defined similarly.
0
lim
x
f x x f x
f x
x
24. If , then the partial derivatives of f with
respect to x and y are the functions and ,
respectively, defined by
and
provided the limits
exists.
,z f x y
xf yf
0
, ,
, limx
x
f x x y f x y
f x y
x
0
, ,
, limy
y
f x y y f x y
f x y
y
25. rahimahj@ump.edu.my
We can interpret partial derivatives as rates of change.
If , then represents the rate of
change of z with respect to x when y is fixed. Similarly,
is the rate of change of z with respect to y when x is
fixed.
),( yxfz xz /
yz /
26. If find and
Solution :
rahimahj@ump.edu.my
,24),( 22
yxyxf )1,1(xf ).1,1(yf
xyxfx 2),(
2)1,1( xf
yyxfy 4),(
4)1,1( yf
EXAMPLE 4
27. rahimahj@ump.edu.my
If calculate and .,sin),(
yx
x
yxf
x
f
y
f
Implicit Differentiation (i)
then,variableoneoffunctionaasfunction
abledifferentiadefinesimplicitly0)If:Theorem
xy
F(x,y
),(
),(
yxF
yxF
dx
dy
y
x
EXAMPLE 5
29. rahimahj@ump.edu.my
Find and if z is defined implicitly as a function of
x and y by the equation
x
z
y
z
054)(
16)(
3222
333
yzzxyzxb
xyzzyxa
Implicit Differentiation (ii)
If z = F (x, y, z) then and
z
x
F
F
x
z
z
y
F
F
y
z
EXAMPLE 7
36. rahimahj@ump.edu.my
If where and ,
find when t = 0 .
,3 42
xyyxz tx 2sin ty cos
dt
dz
Solution :
The Chain Rule gives
we calculate the derivatives, since we get
dt
dy
y
z
dt
dx
x
z
dt
dz
4
32 yxy
x
z
32
12xyx
y
z
,3 42
xyyxz
EXAMPLE 12
37. Then from and , we get
So,
Therefore
rahimahj@ump.edu.my
t
dt
dx
2cos2
tx 2sin ty cos
t
dt
dy
sin
)sin)(12()2cos2)(32( 324
txyxtyxy
dt
dy
y
z
dt
dx
x
z
dt
dz
)sin)(cos2sin122(sin)2cos2)(cos3cos2sin2( 324
tttttttt
6)0)(00()2)(30(
0
tt
z
40. rahimahj@ump.edu.my
The pressure P (in kilopascals), volume V (in liters), and
temperature T (in kelvins) of a mole of an ideal gas are related by
the equation Find the rate at which the pressure is
changing when the temperature is 300 K and increasing at a rate
0.1 K/s and the volume is 100 L and increasing at a rate of 0.2 L/s.
Solution :
If t represent the time elapsed in seconds, then at the given
instant we have
.2.0,100,1.0,300
dt
dV
V
dt
dT
T
.31.8 TPV
EXAMPLE 14
49. rahimahj@ump.edu.my
Objective :
• Compare absolute extrema and local extrema.
• Locate critical points and determine its
classification using second partial derivatives test.
57. A pair (a,b) such that and is
called a critical point or stationary point. To find out
whether a critical point will give f (x, y) a local maximum or
a local minimum, or will give a saddle point, we use
theorem : Second Derivative Test.
0),( bafx
0),( bafy
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“Just believe in yourself and work hard, no
matter what obstacles or hardships come in
your way. You will definitely reach your final
destination.”
66rahimahj@ump.edu.my