Chapter 5: Double Integration
Section 5.6: Changing Variables in a Double Integral
Example 5.6.3
Let R be the first-quadrant region bounded by the curves C1:x2−y2=1, C2:x2−y2=4, C3:x2+y2=9, C4:x2+y2=16.
Use the change of coordinates u=x2−y2,v=x2+y2 to evaluate the Cartesian integral ∫∫Rx y dA.
Solution
Mathematical Solution
Figure 5.6.3(a) shows the region R; Figure 5.6.3(b) shows R′, the image of R under the given change of variables.
Figure 5.6.3(a) Region R
Figure 5.6.3(b) Region R′
Table 5.6.3(a) lists the equations for the mappings between regions R and R′.
Mapping R′→R
Mapping R→R′
x=v+u/2
u=x2−y2
y=v−u/2
v=x2+y2
Table 5.6.3(a) Mappings R↔R′
The Jacobian matrix is 12⁢2⁢v+2⁢u12⁢2⁢v+2⁢u−12⁢−2⁢u+2⁢v12⁢−2⁢u+2⁢v and the Jacobian itself is ∂x,y∂u,v= 14v2−u2, the determinant of the Jacobian matrix. Since fx,y=1, the requisite integral is then
∫∫R′∂x,y∂u,v dv du
= ∫14∫91614v2−u2 ⅆvⅆu
=14⁢ln9+4⁢516+255+ln44+159+65+94⁢arcsin19−arcsin49+4⁢arcsin14−arcsin116
≐0.443319590
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign Name
L=x2−y2=1, x2−y2=4,x2+y2=9,x2+y2=16→assign
U=x2−y2→assign
Context Panel: Assign name
V=x2+y2→assign
Change coordinates and implement the integration in the new coordinate system.
Obtain the equations S=x=xu,v,y=yu,v for the mapping u,v→x,y
Write the equations for the mapping v⁡x,t⁢will now be displayed as⁢v. Press the Enter key.
Context Panel: Solve≻Solve for Variables≻x,y
Context Panel: All Values
Context Panel: Assign to a Name≻S
u=U,v=V
u=x2−y2,v=x2+y2
→solve (specified)
x=RootOf⁡2⁢_Z2−u−v,y=RootOf⁡2⁢_Z2+u−v
→all values
x=v2+u2,y=−u2+v2,x=−v2+u2,y=−u2+v2,x=v2+u2,y=−−u2+v2,x=−v2+u2,y=−−u2+v2
→assign to a name
S
Obtain the Jacobian matrix and the Jacobian
Expression palette: Evaluation template Evaluate x and then y using the information in set S
Context Panel: Evaluate and Display Inline
Context Panel: Assign to a Name≻X or Y, as appropriate
Form the list X,Y Context Panel: Evaluate and Display Inline
Context Panel: Student Multivariate Calculus≻Differentiate≻Jacobian
xx=a|f(x)S1 = v2+u2→assign to a nameX
yx=a|f(x)S1 = −u2+v2→assign to a nameY
X,Y = v2+u2,−u2+v2→Jacobian12⁢2⁢u+2⁢v⁢−2⁢u+2⁢v
Obtain the Jacobian matrix and the Jacobian from first principles
Matrix palette: Insert template for 2×2 matrix.
Calculus palette: Partial derivative operator
Context Panel: Standard Operations≻Determinant
∂∂ u X∂∂ v X∂∂ u Y∂∂ v Y = →determinant12⁢2⁢v+2⁢u⁢−2⁢u+2⁢v
Obtain the images of the edges of the region R
Expression palette: Evaluation template Evaluate the equation of each edge of R using the equations in set S Press the Enter key.
L1x=a|f(x)S1
u=1
L2x=a|f(x)S1
u=4
L3x=a|f(x)S1
v=9
L4x=a|f(x)S1
v=16
Implement the integration over the region R′
Calculus palette: Iterated double-integral template Type in the simplified Jacobian - it is not possible to obtain this form interactively. Press the Enter key.
Context Panel: Approximate≻10 (digits)
∫14∫91614v2−u2 ⅆv ⅆu
14⁢ln⁡9+4⁢5+94⁢arcsin⁡19−14⁢ln⁡16+5⁢3⁢17−4⁢arcsin⁡116−ln⁡9+5⁢13−94⁢arcsin⁡49+2⁢ln⁡2+ln⁡4+5⁢3+4⁢arcsin⁡14
→at 10 digits
0.443319591
Maple Solution - Coded
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
Define the edges of the region defining R.
L≔x2−y2=1, x2−y2=4,x2+y2=9,x2+y2=16:
Define the mapping v⁡x,t⁢will now be displayed as⁢v.
U≔x2−y2:V≔x2+y2:
Apply the solve command. (The first solution maps the first quadrant to the first quadrant, so it is the one to select.)
S≔solveu=U,v=V,x,y,explicit
x=12⁢2⁢v+2⁢u,y=12⁢−2⁢u+2⁢v,x=12⁢2⁢v+2⁢u,y=−12⁢−2⁢u+2⁢v,x=−12⁢2⁢v+2⁢u,y=12⁢−2⁢u+2⁢v,x=−12⁢2⁢v+2⁢u,y=−12⁢−2⁢u+2⁢v
Use the eval command to obtain a list of expressions xu,v,yu,v.
Apply the Jacobian command in its two forms, one to get the matrix, and one to get the determinant.
Simplify the Jacobian by applying the combine and simplify commands under the assumption of positivity, noting that v>u>0.
T≔evalx,y,S1;JacobianT,u,v;J≔simplifycombineJacobianT,u,v,output=determinant assuming positive
12⁢2⁢v+2⁢u,12⁢−2⁢u+2⁢v
Obtain the images of the boundaries of the region R
Use the eval command to make the replacements x→xu,v and y→yu,v in the equation of each edge of the region R.
Use the lhs and rhs commands to move all terms to the left of the resulting equation.
λ≔evalL1,S1:lhsλ−rhsλ=0
u−1=0
λ≔evalL2,S1:lhsλ−rhsλ=0
u−4=0
λ≔evalL3,S1:lhsλ−rhsλ=0
v−9=0
λ≔evalL4,S1:lhsλ−rhsλ=0
v−16=0
Use the Int command to generate the inert integral and apply the value command to evaluate it.
q≔IntJ ,v=9..16,u=1..4;valueq
∫14∫91614⁢−u2+v2ⅆvⅆu
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