James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

1002 Chapter 15 Multiple Integrals
and therefore yy D
f sx, yd dA exists. In particular, this is the case for the following two
types of regions.
A plane region D is said to be of type I if it lies between the graphs of two continuous
functions of x, that is,
D − hsx, yd | a < x < b, t 1sxd < y < t 2 sxdj
where t 1 and t 2 are continuous on fa, bg. Some examples of type I regions are shown in
Figure 5.
y
y=g(x)
y
y=g(x)
y
y=g(x)
D
D
D
y=g¡(x)
y=g¡(x)
y=g¡(x)
0
a
b
x
0
a
b
x
0
a
b
x
FIGURE 5
Some type I regions
y
d
c
0 a x
b x
FIGURE 6
y
d
c
y
d
D
x=h¡(y)
x=h¡(y)
D
y=g(x)
y=g¡(x)
x=h(y)
0 x
0 x
c
FIGURE 7
Some type II regions
D
x=h(y)
In order to evaluate yy D
f sx, yd dA when D is a region of type I, we choose a rectangle
R − fa, bg 3 fc, dg that contains D, as in Figure 6, and we let F be the function
given by Equation 1; that is, F agrees with f on D and F is 0 outside D. Then, by Fubini’s
Theorem,
y
D
y f sx, yd dA − yy Fsx, yd dA − y b
R
a yd c
Fsx, yd dy dx
Observe that Fsx, yd − 0 if y , t 1 sxd or y . t 2 sxd because sx, yd then lies outside D.
Therefore
y d
c
Fsx, yd dy − y t sxd 2
Fsx, yd dy − y t sxd 2
f sx, yd dy
t 1
sxd
because Fsx, yd − f sx, yd when t 1 sxd < y < t 2 sxd. Thus we have the following formula
that enables us to evaluate the double integral as an iterated integral.
3 If f is continuous on a type I region D such that
then
t 1
sxd
D − hsx, yd | a < x < b, t 1sxd < y < t 2 sxdj
y f sx, yd dA − y b sxd 2
a yt f sx, yd dy dx
t 1
sxd
D
The integral on the right side of (3) is an iterated integral that is similar to the ones we
considered in the preceding section, except that in the inner integral we regard x as being
constant not only in f sx, yd but also in the limits of integration, t 1 sxd and t 2 sxd.
We also consider plane regions of type II, which can be expressed as
4 D − hsx, yd | c < y < d, h 1syd < x < h 2 sydj
where h 1 and h 2 are continuous. Two such regions are illustrated in Figure 7.
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