Chapter 8: Applications of Triple Integration
Section 8.5: Changing Variables in a Triple Integral
Example 8.5.3
Using cylindrical coordinates, calculate the volume of R, the region bounded by the paraboloids z=4 x2+y2 and z=9x2+y2 and the planes z=2 and z=3. Then, recalculate the volume by making the change of variables x=u/vcosw, y=u/vsinw, z=u2.
Solution
Mathematical Solution
Figure 8.5.3(a) is a preliminary sketch of the region R, a sketch sufficient to suggest that in cylindrical coordinates, the iteration-order dr dz dθ is one of the simpler approaches, even though it requires expressing the outer (red) surface as rz=z/2, and the inner (green) surface as rz=z/3.
Figure 8.5.3(b) depicts the actual region R whose volume is to be calculated.
Figure 8.5.3(a) Preliminary sketch
Figure 8.5.3(b) The region R
The following iterated integral computes the volume of R in cylindrical coordinates.
∫02 π∫23∫z/3z/2r ⅆr ⅆz ⅆθ
The Jacobian of the transformation x=u/vcosw, y=u/vsinw, z=u2 is
∂x,y,z∂u,v,w=∂uxyz∂vxyz∂wxyz = coswv−u coswv2−u sinwvsinwv−u sinwv2u coswv2 u00 = −2uv3
Since x2+y2=u/v2, the outer surface z=4x2+y2 becomes u2=4u/v2, or v=2. Similarly, the inner surface z=9x2+y2 becomes u2=9u/v2, or v=3. Since z=u2=2 describes the lower plane, u=2 on that surface, while z=u2=3 gives u=3 on the upper plane. Hence, the volume of R can also be calculated via the iterated integral
∫02 π∫23∫232u/v3 ⅆu ⅆv ⅆw
Maple Solution - Interactive
The integration in cylindrical coordinates can be implemented in the task template in Table 8.5.3(a). Note the iteration-order dr dz dθ.
Tools≻Tasks≻Browse: Calculus - Multivariate≻Integration≻Visualizing Regions of Integration≻Cylindrical
Evaluate ∭RΨr,θ,z dv and Graph R
Volume Element dv
r dz dr dθ
r dz dθ dr
r dr dθ dz
r dr dz dθ
r dθ dr dz
r dθ dz dr
, where Ψ=
F=
G=
b=
f=
g=
a=
Table 8.5.3(a) Integration in cylindrical coordinates via a visualization task template
The change of variables and the integration in the resulting coordinate system is implemented in Table 8.5.3(b).
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Assign the transformation equations the name T
Context Panel: Assign to a Name≻T
x=u coswv,y=u sinwv,z=u2→assign to a nameT
Obtain the Jacobian ∂x,y,z∂u,v,w
Expression palette: Evaluation template Press the Enter key.
Context Panel: Jacobian Matrix
Context Panel: Standard Operations≻Determinant
Context Panel: Simplify≻Simplify
x,y,zx=a|f(x)T
u⁢cos⁡wv,u⁢sin⁡wv,u2
→Jacobian
→determinant
−2⁢u3⁢cos⁡w2+sin⁡w2v3
= simplify
−2⁢u3v3
Apply the change of coordinates to the bounding surfaces
Context Panel: Solve≻Solve for Variable≻v
z=9x2+y2x=a|f(x)T
u2=9⁢u2⁢cos⁡w2v2+9⁢u2⁢sin⁡w2v2
u2=9⁢u2v2
→solve for v
v=3,v=−3
z=4x2+y2x=a|f(x)T
u2=4⁢u2⁢cos⁡w2v2+4⁢u2⁢sin⁡w2v2
u2=4⁢u2v2
v=2,v=−2
Context Panel: Solve≻Solve
z=2x=a|f(x)T
u2=2
→solve
u=2,u=−2
z=3x=a|f(x)T
u2=3
u=3,u=−3
Write and evaluate an appropriate iterated triple-integral
Context Panel: Evaluate and Display Inline
∫02 π∫23∫232u/v3 ⅆu ⅆv ⅆw = 2572⁢π
Note the use of the absolute value of the Jacobian.
Maple Solution - Coded
Solution in cylindrical coordinates
Use the Int and int commands.
Intr,r=z/3..z/2,z=2..3,θ=0..2 π= intr,r=z/3..z/2,z=2..3,θ=0..2 π
∫02⁢π∫23∫13⁢z12⁢zrⅆrⅆzⅆθ=2572⁢π
Next, implement a solution in the coordinates defined by the transformation equations
x=u/vcosw, y=u/vsinw, z=u2
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
Let T represent the transformation equations.
T≔x=u coswv,y=u sinwv,z=u2:
Apply the simplify command to the result of a call to the Jacobian command.
simplifyJacobianevalx,y,z,T,u,v,w,output=determinant = −2⁢u3v3
Use the eval command to apply the equations in T to the bounding surfaces.
Use the simplify and solve commands as needed.
s1≔simplifyevalz=4x2+y2,T
solves1,v = 2,−2
s3≔evalz=2,T = u2=2solves3,u2,−2
s2≔simplifyevalz=9x2+y2,T
solves2,v = 3,−3
s4≔evalz=3,T = u2=3solves4,u3,−3
Write and evaluate the appropriate iterated triple-integral
Int2u/v3,u=2..3,v=2..3,w=0..2 π=int2u/v3,u=2..3,v=2..3,w=0..2 π
∫02⁢π∫23∫232⁢u3v3ⅆuⅆvⅆw=2572⁢π
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