Chapter 8: Applications of Triple Integration
Section 8.1: Volume
Example 8.1.11
Use an iterated triple integral to obtain the volume of R, the region bounded by the elliptic paraboloids z=2 x2+5 y2 and z=25− 4 x2−10 y2.
Solution
Mathematical Solution
Figure 8.1.11(a) shows the solid whose volume is obtained by iterating a triple integral in Cartesian coordinates in the order dz dx dy.
∫−5353∫−25−15 y2625−15 y26∫2 x2+5 y225−4 x2−10 y21 dz dx dy = 12512⁢10⁢π
The "glitch" in the surface shown in Figure 8.1.11(a) is an artifact of how Maple draws such surfaces. It is an imperfection in the drawing, not the actual surface.
Figure 8.1.11(a) The region R
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Access the MultiInt command via the Context Panel
a=2 x2+5 y2→assign
b=25−4 x2−10 y2→assign
c=25−15 y2/6→assign
Type the integrand, 1.
Context Panel: Student Multivariate Calculus≻Integrate≻Iterated Fill in the fields of the two dialogs shown below.
Context Panel: Evaluate Integral
1→MultiInt∫−13⁢1513⁢15∫−16⁢−90⁢y2+15016⁢−90⁢y2+150∫2⁢x2+5⁢y2−4⁢x2−10⁢y2+251ⅆzⅆxⅆy=12512⁢2⁢5⁢π
Table 8.1.11(a) provides a solution by a task template that integrates in Cartesian coordinates and draws the region of integration. It helps to make the assignment
Context Panel: Assign to a Name≻X
25−15 y2/6→assign to a nameX
where X is the positive solution of the equation zB=zT, that is, of the equation
2 x2+5 y2=25−4 x2−10 y2
Tools≻Tasks≻Browse: Calculus - Multivariate≻Integration≻Visualizing Regions of Integration≻Cartesian 3-D
Evaluate ∭RΨx,y,z dv and Graph R
Volume Element dv
Select dvdz dy dxdz dx dydx dy dzdx dz dydy dx dzdy dz dx
, where Ψ=
F=
G=
b=
f=
g=
a=
Table 8.1.11(a) Task template integrating in Cartesian coordinates
Table 8.1.11(b) provides a solution from first principles.
Calculus Palette: Iterated triple-integral template
Press the Enter key.
Context Panel: Approximate≻10 (digits)
∫−5353∫−25−15 y2625−15 y26∫2 x2+5 y225−4 x2−10 y21 ⅆz ⅆx ⅆy
12512⁢2⁢5⁢π
→at 10 digits
103.4852944
Table 8.1.11(b) Integration via first principles
Maple Solution - Coded
Make the following definitions so that the necessary integration commands are easier to read.
Define the lower surface.
ZB≔2 x2+5 y2:
Define the upper surface.
ZT≔25−4 x2−10 y2:
Let X be the positive solution of ZB=ZT.
X≔25−15 y2/6:
Table 8.1.11(c) obtains a solution via the MultiInt command in the Student MultivariateCalculus package.
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
MultiInt1,z=ZB..ZT,x=−X..X,y=−5/3..5/3,output=integral
∫−13⁢1513⁢15∫−16⁢−90⁢y2+15016⁢−90⁢y2+150∫2⁢x2+5⁢y2−4⁢x2−10⁢y2+251ⅆzⅆxⅆy
MultiInt1,z=ZB..ZT,x=−X..X,y=−5/3..5/3 = 12512⁢5⁢2⁢π
Table 8.1.11(c) MultiInt command iterating in Cartesian coordinates in the order dy dz dx
Table 8.1.11(d) implements the iterated integration via the top-level Int and int commands.
Int1,z=ZB..ZT,x=−X..X,y=−5/3..5/3=int1,z=ZB..ZT,x=−X..X,y=−5/3..5/3
∫−13⁢1513⁢15∫−16⁢−90⁢y2+15016⁢−90⁢y2+150∫2⁢x2+5⁢y2−4⁢x2−10⁢y2+251ⅆzⅆxⅆy=12512⁢2⁢5⁢π
Table 8.1.11(d) Top-level Int and int commands
Note how Maple rewrites the radicals in the limits of integration.
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