Chapter 5: Double Integration
Section 5.6: Changing Variables in a Double Integral
Example 5.6.5
Let R be the region bounded by the curves C1:16⁢x2⁢y+16⁢y3+8⁢x−88 y=0, C2:144⁢x2⁢y+144⁢y3−24 x−120 y=0, C3:16⁢x2⁢y+16⁢y3−8 x−8 y=0.
Integrate fx,y=x2+y2/y2 over R, noting that it takes two iterations to cover R. Hint: Solve each bounding curve for x=xy and integrate in the order dx dy.
Make the change of variables u=x2+y2, v=x/y and evaluate the integral of f over the image of R under this change of variables.
Solution
Mathematical Solution
Figure 5.6.5(a) shows the region R; Figure 5.6.5(b) shows R′, the image of R under the given change of variables.
Figure 5.6.5(a) Region R
Figure 5.6.5(b) Region R′
Table 5.6.5(a) shows the correspondence between the "corner" points in regions R and R′. Table 5.6.5(b) lists the equations for the mappings between regions R and R′.
Corners in R
Corners in R′
a:1/2,1/2
1,1
b:7/5,1/5
2,7
c:53/26,3/26
3,5
Table 5.6.5(a) Corners in regions R and R′
Mapping R′→R
Mapping R→R′
x=vu1+v2
u=x2+y2
y=u1+v2
v=x/y
Table 5.6.5(b) Mappings R↔R′
Integration over region R is best done by expressing each bounding curve for R in the form x=xy and choosing the order dx dy. Table 5.6.5(c) lists the expressions for the resulting xky,k=1,2,3. The result is
∫32612∫x2x3f dx dy+∫15326∫x2x1f dx dy = 2
x1y=⁢−1+1−16 y4+88⁢y24 y
x2y=⁢1+1−144 y4+120⁢y212 y
x3y=⁢1+1−16 y4+8⁢y24 y
Table 5.6.5(c) Ck solved for x=xy
Direct calculation of the Jacobian ∂x,y∂u,v is tedious. Alternatively, the matrix for the Jacobian ∂u,v∂x,y is 2 x2 y−1/y−x/y2 so this Jacobian is −21+x2/y2. The requisite Jacobian is the reciprocal of this expressed in terms of u and v, that is, ∂x,y∂u,v=−121+v2.
Table 5.6.5(d) lists equations for Ck/ , the image of each Ck in region R′.
Since fxu,v,yu,v=1+v2, the requisite integral is then
C1→v=11−2 u
C2→v=6 u−5
C3→v=2 u−1
Table 5.6.5(d) Images of Ck in R′
∫∫R′fxu,v,yu,v∂x,y∂u,v dv du
= 12∫12∫2 u−16 u−5dv du+12∫23∫2 u−111−2 udv du
=2
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign Name
L=16⁢x2⁢y+16⁢y3+8⁢x−88 y=0,144⁢x2⁢y+144⁢y3−24 x−120 y=0,16 x2⁢y+16⁢y3−8 x−8 y=0→assign
Context Panel: Assign function
fx,y=x2+y2/y2→assign as functionf
U=x2+y2→assign
Context Panel: Assign name
V=x/y→assign
Implement the integration over the region R. Begin by obtaining the coordinates of the "corners" of region R.
Write a sequence of a pair of equations for intersecting edges and press the Enter key.
Context Panel: Solve≻Solve (explicit)
a
L1,L3
16⁢x2⁢y+16⁢y3+8⁢x−88⁢y=0,16⁢x2⁢y+16⁢y3−8⁢x−8⁢y=0
→solve
x=0,y=0,x=526⁢78,y=126⁢78,x=−526⁢78,y=−126⁢78
b
L1,L2
16⁢x2⁢y+16⁢y3+8⁢x−88⁢y=0,144⁢x2⁢y+144⁢y3−24⁢x−120⁢y=0
x=0,y=0,x=75,y=15,x=−75,y=−15
c
Express each edge of region R in the form x=xy.
Write the name of the equation for an edge and press the Enter key.
Context Panel: Solve≻Obtain Solutions for≻x
Context Panel: Select Element≻1
Context Panel: Assign to a Name≻e[k], k=1,2,3
L1
16⁢x2⁢y+16⁢y3+8⁢x−88⁢y=0
→solutions for x
14⁢−1+−16⁢y4+88⁢y2+1y,−14⁢1+−16⁢y4+88⁢y2+1y
→select entry 1
14⁢−1+−16⁢y4+88⁢y2+1y
→assign to a name
e1
L2
144⁢x2⁢y+144⁢y3−24⁢x−120⁢y=0
112⁢1+−144⁢y4+120⁢y2+1y,−112⁢−1+−144⁢y4+120⁢y2+1y
112⁢1+−144⁢y4+120⁢y2+1y
e2
L3
16⁢x2⁢y+16⁢y3−8⁢x−8⁢y=0
14⁢1+−16⁢y4+8⁢y2+1y,−14⁢−1+−16⁢y4+8⁢y2+1y
14⁢1+−16⁢y4+8⁢y2+1y
e3
Write and evaluate the iterated integral.
Calculus palette: Iterated double-integral template
Context Panel: Evaluate and Display Inline
∫3/261/2∫e2e3fx,y ⅆx ⅆy+∫1/53/26∫e2e1fx,y ⅆx ⅆy = 2
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 x,y→u,v. Press the Enter key.
Context Panel: Solve≻Solve for Variables≻x,y
Context Panel: All Values
Select Element≻1
Context Panel: Simplify≻Symbolic
Context Panel: Assign to a Name≻S
u=U,v=V
u=x2+y2,v=xy
→solve (specified)
x=v⁢RootOf⁡v2+1⁢_Z2−u,y=RootOf⁡v2+1⁢_Z2−u
→all values
x=v⁢uv2+1,y=uv2+1,x=−v⁢uv2+1,y=−uv2+1
x=v⁢uv2+1,y=uv2+1
→simplify symbolic
S
Obtain the Jacobian matrix and the Jacobian
Expression palette: Evaluation template Evaluate x and then y using the information in set S
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Form the list X,Y Context Panel: Evaluate and Display Inline
Context Panel: Student Multivariate Calculus≻Differentiate≻Jacobian
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xx=a|f(x)S = v⁢uv2+1→assign to a nameX
yx=a|f(x)S = uv2+1→assign to a nameY
X,Y = v⁢uv2+1,uv2+1→Jacobian−12⁢v2+1→assign to a nameJ
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