Chapter 5: Double Integration
Section 5.6: Changing Variables in a Double Integral
Example 5.6.2
Let R be the interior and boundary of the parallelogram formed by the lines x+y=0, x+y=2, 2 y−3 x=0, 2 y−3 x=7.
Integrate fx,y=x+y2 over R, noting that it takes three iterations to cover R.
Make the change of variables u=x+y, v=2 y−3 x and evaluate the integral of f over the image of R under this change of variables.
Solution
Mathematical Solution
Figure 5.6.2(a) shows the region R; Figure 5.6.2(b) shows R′, the image of R under the given change of variables.
Figure 5.6.2(a) Region R
Figure 5.6.2(b) Region R′
Table 5.6.2(a) gives the coordinates of the vertices of the parallelogram defining region R. Table 5.6.2(b) lists the equations for the mappings between regions R and R′.
Vertex
Coordinates
A
0,0
B
−7/5,7/5
C
−3/5,13/5
D
4/5,6/5
Table 5.6.2(a) Vertices of R
Mapping R′→R
Mapping R→R′
x=2 u−v/5
u=x+y
y=3 u+v/5
v=2 y−3 x
Table 5.6.2(b) Mappings R↔R′
The Jacobian matrix is 2/5−1/53/51/5 and the Jacobian itself is ∂x,y∂u,v= 1/5, the determinant of the Jacobian matrix. Since fxu,v,yu,v=2 u− v/5+3 u+ v/52=u2, the requisite integrals are then
∫−75−35∫−x3 x+72x+y2ⅆyⅆx+∫−350∫−x2−xx+y2ⅆyⅆx+∫045∫32⁢x2−xx+y2ⅆyⅆx = 5615
and
∫∫R′fxu,v,yu,v∂x,y∂u,v dv du = ∫02∫07u25 ⅆvⅆu = 5615
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign Name
L=x+y=0,x+y=2,2 y−3 x=0,2 y−3 x=7→assign
f=x+y2→assign
U=x+y→assign
Context Panel: Assign name
V=2 y−3 x→assign
Obtain the vertices B, C, D of the parallelogram forming the region R
Make a list of two intersecting edges and press the Enter key.
Context Panel: Evaluate and Display Inline
Context Panel: Solve≻Solve
Context Panel: Assign to a Name≻b,c,d, as appropriate
L1,L4 = x+y=0,2⁢y−3⁢x=7→solvex=−75,y=75→assign to a nameb
L4,L2 = 2⁢y−3⁢x=7,x+y=2→solvex=−35,y=135→assign to a namec
L2,L3 = x+y=2,2⁢y−3⁢x=0→solvex=45,y=65→assign to a named
Implement the integration over the region R
Calculus palette: Iterated double-integral template
∫−7/5−3/5∫−x3 x+7/2f ⅆy ⅆx+∫−3/50∫−x2−xf ⅆy ⅆx+∫04/5∫3 x/22−xf ⅆy ⅆx = 5615
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: Assign to a Name≻S
u=U,v=V
u=x+y,v=2⁢y−3⁢x
→solve (specified)
x=2⁢u5−v5,y=v5+3⁢u5
→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: 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)S = 2⁢u5−v5→assign to a nameX
yx=a|f(x)S = v5+3⁢u5→assign to a nameY
X,Y = 2⁢u5−v5,v5+3⁢u5→Jacobian15
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 = →determinant15
Obtain the images of the edges of the parallelogram defining 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)S
u=0
L2x=a|f(x)S
u=2
L3x=a|f(x)S
v=0
L4x=a|f(x)S
v=7
Transform the integrand
Expression palette: Evaluation template Evaluate f using the equations in set S.
Context Panel: Assign to a Name≻F
fx=a|f(x)S = u2→assign to a nameF
Implement the integration over the region R′
∫02∫07F/5 ⅆv ⅆu = 5615
Maple Solution - Coded
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
Define the edges of the parallelogram defining R.
L≔x+y=0,x+y=2,2 y−3 x=0,2 y−3 x=7:
Define the integrand f.
f≔x+y2:
Define the mapping v⁡x,t⁢will now be displayed as⁢v.
U≔x,y→x+y:V≔x,y→2 y−3 x:
Apply the solve command, obtaining the coordinates of B, C, and D. Clearly, the coordinates of A are 0,0.
solveL1,L4 = x=−75,y=75
solveL4,L2 = x=−35,y=135
solveL2,L3 = x=45,y=65
Use the Int command to generate the inert integral and apply the value command to evaluate it.
Q≔Intf,y=−x..3 x+7/2,x=−7/5..−3/5+Intf,y=−x..2−x,x=−3/5..0+Intf,y=3 x/2..2−x,x=0..4/5; valueQ
∫−75−35∫−x32⁢x+72x+y2ⅆyⅆx+∫−350∫−x2−xx+y2ⅆyⅆx+∫045∫32⁢x2−xx+y2ⅆyⅆx
Apply the solve command.
S≔solveu=Ux,y,v=Vx,y,x,y
x=25⁢u−15⁢v,y=15⁢v+35⁢u
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.
T≔evalx,y,S;JacobianT,u,v;JacobianT,u,v,output=determinant
25⁢u−15⁢v,15⁢v+35⁢u
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.
temp≔evalL1,S:lhstemp−rhstemp=0
temp≔evalL2,S:lhstemp−rhstemp=0
u−2=0
temp≔evalL3,S:lhstemp−rhstemp=0
temp≔evalL4,S: lhstemp−rhstemp=0
v−7=0
Use the eval command to make the replacements x→xu,v and y→yu,v in fx,y.
F≔evalf,S
u2
q≔Int F/5,v=0..7,u=0..2;valueq
∫02∫0715⁢u2ⅆvⅆu
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