Chapter 8: Applications of Triple Integration
Section 8.1: Volume
Example 8.1.16
Use an iterated triple integral to obtain the volume of R, the region that lies inside the sphere x2+y2+z2=4, and is between the cones z=x2+y2 and z=3x2+y2.
Solution
Mathematical Solution
Figure 8.1.16(a) shows the solid whose volume is obtained by iterating a triple integral in spherical coordinates in the order dρ dφ dθ.
∫02 π∫π/6π/4∫02ρ2sinφ dρ dφ dθ = 83π 3−2
In spherical coordinates, the cone z=x2+y2 is described by φ=π/4, whereas the cone z=3x2+y2is described by π/6.
This latter angle can be determined by converting the Cartesian equation to spherical coordinates:
Figure 8.1.16(a) Region R
z
= 3x2+y2
z2
ρ cosφ2
= 3ρ sinφcosθ2+ρ sinφsinθ2
ρ2cos2φ
= 3 ρ2sin2φ cos2θ+sin2θ
cos2φ
= 3 sin2φ
from which it follows that tanφ=1/3 and φ=π/6.
Maple Solution - Interactive
Table 8.1.16(a) provides a solution by a task template that integrates in cylindrical coordinates and draws the region of integration.
Tools≻Tasks≻Browse: Calculus - Multivariate≻Integration≻Visualizing Regions of Integration≻Spherical
Evaluate ∭RΨρ,φ,θ dv and Graph R
Volume Element dv=ρ2sinφ×
dρ dφ dθ
dρ dθ dφ
dφ dρ dθ
dφ dθ dρ
dθ dφ dρ
dθ dρ dφ
, where Ψ=
F=
G=
b=
f=
g=
a=
Table 8.1.16(a) Task template integrating in spherical coordinates
(The outer cone was made transparent via the Context Panel for graph in the task template.)
Since the iteration order can be taken as dρ dφ dθ, the task template in Table 8.1.16(a), using the MultiInt command from the Student MultivariateCalculus package, applies.
Tools≻Tasks≻Browse: Calculus - Multivariate≻Integration≻Multiple Integration≻Spherical
Iterated Triple Integral in Spherical Coordinates
(φ = colatitude, measured down from z-axis)
Integrand:
1
Region: ρ1φ,θ≤ρ≤ρ2φ,θ,φ1θ≤φ≤φ2θ,a≤θ≤b
ρ1φ,θ
0
ρ2φ,θ
2
φ1θ
π/6
16⁢π
φ2θ
π/4
14⁢π
a
b
2 π
2⁢π
Inert Integral: dρ dφ dθ
StudentMultivariateCalculusMultiInt,ρ=..,φ=..,θ=..,coordinates=sphericalρ,φ,θ,output=integral
∫02⁢π∫16⁢π14⁢π∫02ρ2⁢sin⁡φⅆρⅆφⅆθ
Value:
StudentMultivariateCalculusMultiInt,ρ=..,φ=..,θ=.., coordinates=sphericalρ,φ,θ
83⁢3⁢π−83⁢2⁢π
Stepwise Evaluation:
StudentMultivariateCalculusMultiInt,ρ=.., φ=..,θ=..,coordinates=sphericalρ,φ,θ,output=steps
∫02⁢π∫π6π4∫02ρ2⁢sin⁡φⅆρⅆφⅆθ=∫02⁢π∫π6π4ρ3⁢sin⁡φ3ρ=0..2|ρ3⁢sin⁡φ3ρ=0..2ⅆφⅆθ=∫02⁢π∫π6π48⁢sin⁡φ3ⅆφⅆθ=∫02⁢π−8⁢cos⁡φ3φ=π6..π4|−8⁢cos⁡φ3φ=π6..π4ⅆθ=∫02⁢π4⁢33−4⁢23ⅆθ=4⁢33−4⁢23⁢θθ=0..2⁢π|4⁢33−4⁢23⁢θθ=0..2⁢π
Table 8.1.16(b) Task template implementing the MultiInt command iterating in the order dρ dφ dθ
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∫02⁢π∫16⁢π14⁢π∫02ρ2⁢sin⁡φⅆρⅆφⅆθ=83⁢π⁢3−83⁢2⁢π
Table 8.1.16(c) provides a solution from first principles.
Calculus Palette: Iterated triple-integral template
Context Panel: Evaluate and Display Inline
∫02 π∫π/6π/4∫02ρ2sinφ ⅆρ ⅆφ ⅆθ = 83⁢3⁢π−83⁢2⁢π
Table 8.1.16(c) Integration via first principles
Maple Solution - Coded
Table 8.1.16(d) obtains a solution via the MultiInt command in the Student MultivariateCalculus package. See Table 8.1.16(b) for an implementation of this integration via a task template.
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
MultiInt1,ρ=0..2,φ=π/6..π/4,θ=0..2 π,coordinates=sphericalρ,φ,θ,output=integral
MultiInt1,ρ=0..2,φ=π/6..π/4,θ=0..2 π,coordinates=sphericalρ,φ,θ
Table 8.1.16(d) MultiInt command iterating in spherical coordinates in the order dρ dφ dθ
Table 8.1.16(e) implements the iterated integration via the top-level Int and int commands.
Intρ2sinφ,ρ=0..2,φ=π/6..π/4,θ=0..2 π=intρ2sinφ,ρ=0..2,φ=π/6..π/4,θ=0..2 π
∫02⁢π∫16⁢π14⁢π∫02ρ2⁢sin⁡φⅆρⅆφⅆθ=163⁢12⁢3−12⁢2⁢π
Table 8.1.16(e) Top-level Int and int commands
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