Chapter 5: Double Integration
Section 5.6: Changing Variables in a Double Integral
|
Example 5.6.5
|
|
Let be the region bounded by the curves , , .
a)
|
Integrate over , noting that it takes two iterations to cover . Hint: Solve each bounding curve for and integrate in the order .
|
b)
|
Make the change of variables , and evaluate the integral of over the image of under this change of variables.
|
|
|
|
|
Solution
|
|
|
Mathematical Solution
|
|
•
|
Figure 5.6.5(a) shows the region ; Figure 5.6.5(b) shows , the image of under the given change of variables.
|
|
Figure 5.6.5(a) Region
|
|
|
|
|
Figure 5.6.5(b) Region
|
|
|
|
|
|
•
|
Table 5.6.5(a) shows the correspondence between the "corner" points in regions and . Table 5.6.5(b) lists the equations for the mappings between regions and .
|
Corners in
|
Corners in
|
|
|
|
|
|
|
Table 5.6.5(a) Corners in regions and
|
|
|
|
Mapping
|
Mapping
|
|
|
|
|
Table 5.6.5(b) Mappings
|
|
|
|
|
|
•
|
Integration over region is best done by expressing each bounding curve for in the form and choosing the order . Table 5.6.5(c) lists the expressions for the resulting . The result is
|
= 2
|
|
|
|
Table 5.6.5(c) solved for
|
|
|
|
|
|
•
|
Direct calculation of the Jacobian is tedious. Alternatively, the matrix for the Jacobian is so this Jacobian is . The requisite Jacobian is the reciprocal of this expressed in terms of and , that is, .
|
•
|
Table 5.6.5(d) lists equations for , the image of each in region .
|
•
|
Since , the requisite integral is then
|
|
|
|
|
Table 5.6.5(d) Images of in
|
|
|
|
|
|
|
|
Maple Solution - Interactive
|
|
Initialize
|
•
|
Tools≻Load Package:
Student Multivariate Calculus
|
|
Loading Student:-MultivariateCalculus
|
•
|
Context Panel: Assign Name
|
|
|
•
|
Context Panel: Assign function
|
|
|
•
|
Context Panel: Assign Name
|
|
|
•
|
Context Panel: Assign name
|
|
|
|
|
Implement the integration over the region . Begin by obtaining the coordinates of the "corners" of region .
•
|
Write a sequence of a pair of equations for intersecting edges and press the Enter key.
|
•
|
Context Panel: Solve≻Solve (explicit)
|
|
|
|
|
|
|
|
|
|
Express each edge of region in the form .
•
|
Write the name of the equation for an edge and press the Enter key.
|
•
|
Context Panel: Solve≻Obtain Solutions for≻
|
•
|
Context Panel: Select Element≻1
|
•
|
Context Panel: Assign to a Name≻e[k],
|
|
|
|
|
|
|
Write and evaluate the iterated integral.
•
|
Calculus palette: Iterated double-integral template
|
•
|
Context Panel: Evaluate and Display Inline
|
|
=
|
|
|
Change coordinates and implement the integration in the new coordinate system.
Obtain the equations for the mapping
|
•
|
Write the equations for the mapping . Press the Enter key.
|
•
|
Context Panel: Solve≻Solve for Variables≻
|
•
|
Context Panel: All Values
|
•
|
Context Panel: Simplify≻Symbolic
|
•
|
Context Panel: Assign to a Name≻
|
|
|
Obtain the Jacobian matrix and the Jacobian
|
•
|
Expression palette: Evaluation template
Evaluate and then using the information in set
|
•
|
Context Panel: Evaluate and Display Inline
|
•
|
Context Panel: Assign to a Name≻ or , as appropriate
|
•
|
Form the list
Context Panel: Evaluate and Display Inline
|
•
|
Context Panel: Student Multivariate Calculus≻Differentiate≻Jacobian
|
•
|
Context Panel: Assign to a Name≻
|
|
=
|
=
|
=
|
|
Obtain the Jacobian matrix and the Jacobian from first principles
|
•
|
Matrix palette: Insert template for matrix.
|
•
|
Calculus palette: Partial derivative operator
|
•
|
Context Panel: Evaluate and Display Inline
|
•
|
Context Panel: Standard Operations≻Determinant
|
|
=
|
Obtain the images of the edges of the region
|
•
|
Expression palette: Evaluation template
Evaluate the equation of each edge of using the equations in set .
Press the Enter key.
|
•
|
Context Panel: Solve≻Isolate Expression for≻
|
|
|
Obtain the transformed integrand
|
•
|
Context Panel: Simplify≻Assuming Real
|
|
=
|
Implement the integration over the region
|
•
|
Calculus palette:
Iterated double-integral template
|
•
|
Context Panel: Evaluate and Display Inline
|
|
=
|
|
|
|
|
Maple Solution - Coded
|
|
Initialize
|
•
|
Install the Student MultivariateCalculus package.
|
|
|
|
|
•
|
Define the edges of the region defining .
|
|
|
|
|
|
|
Integration over region is best done in the order , and this is facilitated by expressing each bounding curve in the form . This is done in Table 5.6.5(e).
|
=
|
|
=
|
|
=
|
Table 5.6.5(e) Each in solved for
|
|
|
The corners of region are found by pairwise intersecting the bounding curves . This is done in Table 5.6.5(f), and correlates with Figure 5.6.5(a)
Point
|
Coordinates
|
|
|
|
|
|
|
Table 5.6.5(f) Corners of region : Select appropriate solution based on Figure 5.6.5(a)
|
|
|
The iteration over is then
=
Next, change coordinates and implement the integration in the new coordinate system.
Obtain the equations for the mapping
|
•
|
Apply the solve command and simplify it under the assumption of positivity.
(The first solution maps the first quadrant to the first quadrant, so it is the one to select.)
|
|
|
Obtain the Jacobian matrix and the Jacobian
|
•
|
Use the eval command to obtain a list of expressions .
|
•
|
Apply the Jacobian command in its two forms, one to get the matrix, and one to get the determinant.
Apply the simplify command to the Jacobian matrix.
|
|
|
Obtain the images of the boundaries of the region
|
•
|
Use the eval command to make the replacements and in the equation of each edge of the region .
|
•
|
Use the isolate command to obtain equations of the form
|
|
|
Obtain the transformed integrand
|
•
|
Apply the simplify command, where and .
|
|
=
|
Implement the integration over the region
|
•
|
Use the Int command to generate the inert integral and apply the value command to evaluate it.
|
|
|
|
|
|
|
|
<< Previous Example Section 5.6
Next Section >>
© Maplesoft, a division of Waterloo Maple Inc., 2024. All rights reserved. This product is protected by copyright and distributed under licenses restricting its use, copying, distribution, and decompilation.
For more information on Maplesoft products and services, visit www.maplesoft.com
|