Chapter 5: Double Integration
Section 5.3: Regions with Curved Boundaries
Example 5.3.1
Integrate fx,y=1+2 x2+3 y2 over the region R={x,y | 0≤x≤1,x2≤y≤x}.
Solution
Mathematical Solution
To iterate in the order dy dx, describe the bounding curves as in Figure 5.3.1(a) where yB=x2 is the lower limit of the inner integral, and yT=x is the upper limit.
To iterate in the order dx dy, describe the bounding curves as in Figure 5.3.1(b) where xL=y2 is the lower limit of the inner integral, and xR=y is the upper limit.
Figure 5.3.1(a) Iterating in the order dy dx
Figure 5.3.1(b) Iterating in the order dx dy
The resulting iterated integrals are therefore
∫01∫x2x1+2 x2+3 y2 ⅆy ⅆx = 1621 = ∫01∫y2y1+2 x2+3 y2 ⅆx ⅆy
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Context Panel: Assign Name
f=1+2 x2+3 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∫01∫x2x2⁢x2+3⁢y2+1ⅆyⅆx=1621
Tables 5.3.1(a, b) contain a visualization task template wherein a double integral can be iterated in either order. In the drop-down for area-element dA, select an ordering for the iteration. Then fill in the remaining fields as shown.
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 5.3.1(a) Visualizing R and the resulting volume for iteration in the order dy dx
The vertical arrow in the left-hand graph indicates that the iteration is in the order dy dx, whereby the first (or inner) integration is in the vertical direction, from the lowermost boundary curve to the uppermost. 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.
Table 5.3.1(b) shows the iteration in the opposite order.
Table 5.3.1(b) Visualizing R and the resulting volume for iteration in the order dx dy
When integrating in the order dx dy, the first (or inner) integral is taken in the horizontal direction, as indicated by the arrow in the left-hand graph.
The detailed analytic results below are obtained via the palettes and Context Panel.
Iterate dy dx and dx dy
Calculus palette: Template for definite iterated double integral
Context Panel: Evaluate and Display Inline
∫01∫x2xf ⅆy ⅆx = 1621
∫01∫y2yf ⅆx ⅆy = 1621
Display the iterated integrals
Calculus palette: Template for definite iterated double integral Context Panel: 2-D Math≻Convert To≻Inert Form Press the Enter key
∫01∫x2xf ⅆy ⅆx
∫01∫x2x2⁢x2+3⁢y2+1ⅆyⅆx
=
1621
∫01∫y2yf ⅆx ⅆy
∫01∫y2y2⁢x2+3⁢y2+1ⅆxⅆy
Maple Solution - Coded
Install the Student MultivariateCalculus package.
withStudent:-MultivariateCalculus:
Define the integrand.
f≔1+2 x2+3 y2:
Iterate in the order dy dx
Use the Int command to obtain the inert integral and the int command for immediate evaluation
Intf,y=x2..x,x=0..1=intf,y=x2..x,x=0..1
∫01∫x2x2⁢x2+3⁢y2+1ⅆyⅆx=1621
Iterate in the order dx dy
Intf,x=y2..y,y=0..1=intf,x=y2..y,y=0..1
∫01∫y2y2⁢x2+3⁢y2+1ⅆxⅆy=1621
Use the MultiInt command from the Student MultivariateCalculus package
MultiIntf,y=x2..x,x=0..1,output=integral
MultiIntf,y=x2..x,x=0..1
MultiIntf,x=y2..y,y=0..1,output=integral
MultiIntf,x=y2..y,y=0..1
Obtain stepwise evaluations via the MultiInt command
MultiIntf,y=x2..x,x=0..1,output=steps
∫01∫x2x2⁢x2+3⁢y2+1ⅆyⅆx=∫012⁢x2⁢y+y3+yy=x2..x|2⁢x2⁢y+y3+yy=x2..xⅆx=∫012⁢x2⁢x−x2+x32−x6+x−x2ⅆx=−2⁢x55+4⁢x727+2⁢x525−x77+2⁢x323−x33x=0..1|−2⁢x55+4⁢x727+2⁢x525−x77+2⁢x323−x33x=0..1
MultiIntf,x=y2..y,y=0..1,output=steps
∫01∫y2y2⁢x2+3⁢y2+1ⅆxⅆy=∫0123⁢x3+3⁢y2⁢x+xx=y2..y|23⁢x3+3⁢y2⁢x+xx=y2..yⅆy=∫012⁢y323−2⁢y63+3⁢y2⁢y−y2+y−y2ⅆy=4⁢y5215−2⁢y721−3⁢y55+6⁢y727+2⁢y323−y33y=0..1|4⁢y5215−2⁢y721−3⁢y55+6⁢y727+2⁢y323−y33y=0..1
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