Areas of Bounded Regions in the Plane

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Question 1: Summarize the Riemann sum argument that gives the formula of area as a double integral.

Hint 1: Read the subsection “Areas of Bounded Regions in the Plane” (p. 911 – 913). 

15.3 Area by Double Integration
In this section we show how to use double integrals to calculate the areas of bounded
regions in the plane, and to find the average value of a function of two variables.
Areas of Bounded Regions in the Plane
If we take f(x, y) = 1 in the definition of the double integral over a region R in the preced-
ing section, the Riemann sums reduce to
S₁₁ =
FOX. 30 AA - AA.
(1)
This is simply the sum of the areas of the small rectangles in the partition of R, and
approximates what we would like to call the area of R. As the norm of a partition of R
approaches zero, the height and width of all rectangles in the partition approach zero, and
the coverage of R becomes increasingly complete (Figure 15.8). We define the area of R to
be the limit
ο ΣΔΑ
lim
||P0
R
dA.
(2)
Transcribed Image Text:15.3 Area by Double Integration In this section we show how to use double integrals to calculate the areas of bounded regions in the plane, and to find the average value of a function of two variables. Areas of Bounded Regions in the Plane If we take f(x, y) = 1 in the definition of the double integral over a region R in the preced- ing section, the Riemann sums reduce to S₁₁ = FOX. 30 AA - AA. (1) This is simply the sum of the areas of the small rectangles in the partition of R, and approximates what we would like to call the area of R. As the norm of a partition of R approaches zero, the height and width of all rectangles in the partition approach zero, and the coverage of R becomes increasingly complete (Figure 15.8). We define the area of R to be the limit ο ΣΔΑ lim ||P0 R dA. (2)
912
0
Chapter 15 Multiple Integrals
(-1, 1),
FIGURE 15.19 The region in
Example 1.
R₁
0
(1, 1)
/y=x?
(a)
y=x+2,
(2,4)
for de
R₂
dx dy
dx dy
Chapter 15 Multiple Integrals
DEFINITION The area of a closed, bounded plane region R is
- [[
R
As with the other definitions in this chapter, the definition here applies to a greater
variety of regions than does the earlier single-variable definition of area, but it agrees with
the earlier definition on regions to which they both apply. To evaluate the integral in the
definition of area, we integrate the constant function f(x, y) = 1 over R.
A =
EXAMPLE 1 Find the area of the region R bounded by y = x and y = x² in the first
quadrant.
1- [Say
dA.
Solution We sketch the region (Figure 15.19), noting where the two curves intersect at
the origin and (1, 1), and calculate the area as
EXAMPLE 2
line y = x + 2.
dy dx =
- - L'[ - ], ₁²₁ - L' (x
dx =
(x - x²) dx =
31-
3
6
Notice that the single-variable integral f(x-x²) dx, obtained from evaluating the inside
iterated integral, is the integral for the area between these two curves using the method of
Section 5.6.
Find the area of the region R enclosed by the parabola y = ² and the
Solution If we divide R into the regions R, and R₂ shown in Figure 15.20a, we may
calculate the area as
Transcribed Image Text:912 0 Chapter 15 Multiple Integrals (-1, 1), FIGURE 15.19 The region in Example 1. R₁ 0 (1, 1) /y=x? (a) y=x+2, (2,4) for de R₂ dx dy dx dy Chapter 15 Multiple Integrals DEFINITION The area of a closed, bounded plane region R is - [[ R As with the other definitions in this chapter, the definition here applies to a greater variety of regions than does the earlier single-variable definition of area, but it agrees with the earlier definition on regions to which they both apply. To evaluate the integral in the definition of area, we integrate the constant function f(x, y) = 1 over R. A = EXAMPLE 1 Find the area of the region R bounded by y = x and y = x² in the first quadrant. 1- [Say dA. Solution We sketch the region (Figure 15.19), noting where the two curves intersect at the origin and (1, 1), and calculate the area as EXAMPLE 2 line y = x + 2. dy dx = - - L'[ - ], ₁²₁ - L' (x dx = (x - x²) dx = 31- 3 6 Notice that the single-variable integral f(x-x²) dx, obtained from evaluating the inside iterated integral, is the integral for the area between these two curves using the method of Section 5.6. Find the area of the region R enclosed by the parabola y = ² and the Solution If we divide R into the regions R, and R₂ shown in Figure 15.20a, we may calculate the area as
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