Chapter 8: Applications of Triple Integration
Section 8.1: Volume
Example 8.1.2
Use an iterated triple integral to obtain the volume of the region R, the tetrahedron cut from the first octant by the plane 3 x+5 y+7 z=15.
Solution
Mathematical Solution
Figure 8.1.2(a) shows the tetrahedron whose volume is obtained by iterating a triple integral in Cartesian coordinates in the order dz dy dx.
∫05∫031−x5∫015−3 x−5 y71 dz dy dx = 7514
Figure 8.1.2(a) Tetrahedron
Maple Solution - Interactive
Since the iteration order can be taken as dz dy dx, the task template in Table 8.1.2(a), using the MultiInt command from the Student MultivariateCalculus package, applies.
Tools≻Tasks≻Browse: Calculus - Multivariate≻Integration≻Multiple Integration≻Cartesian 3-D
Iterated Triple Integrals in Cartesian Coordinates
Integrand:
1
Region: z1x,y≤z≤z2x,y,y1x≤y≤y2x,a≤x≤b
z1x,y
0
z2x,y
(15−3 x−5 y)/7
157−37⁢x−57⁢y
y1x
y2x
31−x/5
3−35⁢x
a
b
5
Inert Integral: dz dy dx
StudentMultivariateCalculusMultiInt,z=..,y=..,x=..,output=integral
∫05∫03−35⁢x∫0157−37⁢x−57⁢y1ⅆzⅆyⅆx
Value:
StudentMultivariateCalculusMultiInt,z=..,y=..,x=..
7514
Stepwise Evaluation:
StudentMultivariateCalculusMultiInt,z=..,y=..,x=..,output=steps
∫05∫03−3⁢x5∫0157−3⁢x7−5⁢y71ⅆzⅆyⅆx=∫05∫03−3⁢x5zz=0..157−3⁢x7−5⁢y7|zz=0..157−3⁢x7−5⁢y7ⅆyⅆx=∫05∫03−3⁢x5157−3⁢x7−5⁢y7ⅆyⅆx=∫05157⁢y−37⁢x⁢y−514⁢y2y=0..3−3⁢x5|157⁢y−37⁢x⁢y−514⁢y2y=0..3−3⁢x5ⅆx=∫05457−9⁢x7−3⁢x⁢3−3⁢x57−5⁢3−3⁢x5214ⅆx=45⁢x7−9⁢x27+3⁢x335+25⁢3−3⁢x53126x=0..5|45⁢x7−9⁢x27+3⁢x335+25⁢3−3⁢x53126x=0..5
Table 8.1.2(a) Task template implementing the MultiInt command iterating in the order dz dy dx
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Access the MultiInt command via the Context Panel
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∫05∫03−35⁢x∫0157−37⁢x−57⁢y1ⅆzⅆyⅆx=7514
Table 8.1.2(b) provides a solution from first principles.
Calculus Palette: Iterated triple-integral template
Context Panel: Evaluate and Display Inline
∫05∫031−x/5∫015−3 x−5 y/71 ⅆz ⅆy ⅆx = 7514
Table 8.1.2(b) Integration via first principles
Maple Solution - Coded
Table 8.1.2(c) provides a solution via the command the MultiInt command in the Student MultivariateCalculus package. This command recognizes the tetrahedron as a predefined region that is specified by the four vertices of the tetrahedron.
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
V≔0,0,0,5,0,0,0,3,0,0,0,15/7:MultiInt1,x,y,z=TetrahedronV,output=integral
157⁢∫01∫05−5⁢t∫0−3+3⁢t⁢x−5+5⁢t5−5⁢t1ⅆyⅆxⅆt
MultiInt1,x,y,z=TetrahedronV = 7514
Table 8.1.2(c) Integration over a tetrahedron
Table 8.1.2(d) obtains a more primitive solution via the MultiInt command in the Student MultivariateCalculus package. See Table 8.1.2(a) for an implementation of this command via a task template.
Student:-MultivariateCalculus:-MultiInt1,z=0..15−3 x−5 y/7,y=0..31−x/5,x=0..5
Table 8.1.2(d) MultiInt command iterating in the order dz dy dx
Table 8.1.2(e) implements the iterated integration via the top-level Int and int commands.
Int1,z=0..15−3 x−5 y/7,y=0..31−x/5,x=0..5=int1,z=0..15−3 x−5 y/7,y=0..31−x/5,x=0..5
∫05∫03−35⁢x∫0157−37⁢x−57⁢y1ⅆzⅆyⅆx=7514
Table 8.1.2(e) Top-level Int and int commands
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