Chapter 5: Double Integration
Section 5.3: Regions with Curved Boundaries
Example 5.3.5
Integrate fx,y=1+2 x2+3 y2 over the region R={x,y | 1−y2≤x≤4−1−y2,−1≤y≤1}.
Solution
Mathematical Solution
To iterate in the order dx dy, describe the bounding curves as in Figure 5.3.5(a) where xL=1−y2 is the lower limit of the inner integral, and xR=4−1−y2 is the upper limit.
Figure 5.3.5(b) suggests that iteration in the order dy dx requires five different doubly iterated integrals.
Figure 5.3.5(a) Iterating in the order dx dy
use plots in module() local q1,q2,p1,p2,p3,p4,p5,p6; q1:=sqrt(1-x^2): q2:=sqrt(1-(x-4)^2): p1:=shadebetween(1,q1,x=0..1,color=red): p2:=shadebetween(-q1,-1,x=0..1,color=green): p3:=shadebetween(-1,1,x=1..3,color=gray): p4:=shadebetween(q2,1,x=3..4,color=blue): p5:=shadebetween(-q2,-1,x=3..4,color=gold): p6:=display(p1,p2,p3,p4,p5,scaling=constrained,labels=[x,y],tickmarks=[5,3]); print(p6); end module: end use:
Figure 5.3.5(b) Iterating in the order dy dx
Table 5.3.5(a) shows these integrations whose sum is 112−73 π/4≐54.67.
Region
Iterated Integral
red
∫01∫1−x211+2 x2+3 y2 ⅆy ⅆx = 83−916⁢π
green
∫01∫−1−1−x21+2 x2+3 y2 ⅆy ⅆx = 83−916⁢π
blue
∫34∫1−x−4211+2 x2+3 y2 ⅆy ⅆx = 32−13716⁢π
gold
∫34∫−1−1−x−421+2 x2+3 y2 ⅆy ⅆx = 32−13716⁢π
gray
∫13∫−111+2 x2+3 y2 ⅆy ⅆx = 1283
Table 5.3.5(a) Five iterated integrals for the integration order dy dx
Iterating in the order dx dy results in the much simpler
∫−11∫1−y24−1−y21+2 x2+3 y2 ⅆx ⅆy = 112−734⁢π ≐ 54.67
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign Name
f=1+2 x2+3 y2→assign
XL=1−y2→assign
XR=4−1−y2→assign
Access the MultiInt command via the Context Panel
Write f, the name of the integrand. Context Panel: Evaluate and Display Inline
Context Panel: Student Multivariate Calculus≻Integrate≻Iterated Fill in both panes (see Figures 5.3.(1, 2)) and select "integral" for the Output
Context Panel: Evaluate Integral
f = 2⁢x2+3⁢y2+1→MultiInt∫−11∫−y2+14−−y2+12⁢x2+3⁢y2+1ⅆxⅆy=112−734⁢π
Table 5.3.5(b) illustrates the visualization task template keyed to iterate in the order dx dy.
Tools≻Tasks≻Browse:
Calculus - Multivariate≻Integration≻Visualizing Regions of Integration≻
Evaluate ∬RΨx,y dA and Graph R
Area Element dA
Select dAdy dxdx dy
, Ψ=
Value of Integral
G=
b=
g=
a=
Bounding Curves
"Volume"
Table 5.3.5(b) Visualizing R and the resulting volume for iteration in the order dy dx
The horizontal arrow in the left-hand graph indicates that the iteration is in the order dx dy, whereby the first (or inner) integration is in the horizontal direction, from the leftmost boundary curve to the rightmost. Because the integrand is positive, the double integral calculates the volume below the surface z=f but above the plane z=0. The solid whose volume is thereby calculated is seen in the right-hand graph.
The detailed analytic results below are obtained via the palettes and Context Panel.
Iterate in the order dy dx
Calculus palette: Template for definite iterated double integral
Context Panel: Evaluate and Display Inline
∫−11∫1−y24−1−y2f ⅆx ⅆy = 112−734⁢π
Display the iterated integrals
Context Panel: 2-D Math≻Convert To≻Inert Form
Press the Enter key
∫−11∫1−y24−1−y2f ⅆx ⅆy
∫−11∫−y2+14−−y2+12⁢x2+3⁢y2+1ⅆxⅆy
=
112−734⁢π
Maple Solution - Coded
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
Define the integrand.
f≔1+2 x2+3 y2:
Iterate in the order dx dy
Use the Int command to obtain the inert integral and the int command for immediate evaluation
Intf,x=1−y2..4−1−y2,y=−1..1=intf,x=1−y2..4−1−y2,y=−1..1
∫−11∫−y2+14−−y2+12⁢x2+3⁢y2+1ⅆxⅆy=112−734⁢π
Use the MultiInt command from the Student MultivariateCalculus package
MultiIntf,x=1−y2..4−1−y2,y=−1..1,output=integral
MultiIntf,x=1−y2..4−1−y2,y=−1..1
Obtain stepwise evaluations via the MultiInt command
MultiIntf,x=1−y2..4−1−y2,y=−1..1,output=steps:
∫−11∫−y2+14−−y2+12⁢x2+3⁢y2+1ⅆxⅆy
=112−734⁢π
The stepwise evaluation of the iterated integral is inserted as an image because the actual display does not fit comfortably in the available space.
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