Chapter 6: Applications of Double Integration
Section 6.4: Average Value
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Essentials
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The average value of on the interval is given by .
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Similarly, the average value of on a region of area is given by
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In polar coordinates, the average value of on a region of area is given by
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where now, includes , the absolute value of the Jacobian of the transformation from polar to Cartesian coordinates.
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Figure 6.4.1 shows the average value of on , the first-quadrant portion of the unit circle.
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The average value is given by
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However, there is as much volume above the plane as there is below.
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use plots in
module()
local p1,p2,p3;
p1:=plot3d(x^2+y^2,x=0..1,y=0..sqrt(1-x^2),filled=true):
p2:=plot3d(1/2,x=0..1,y=0..sqrt(1-x^2)):
p3:=display(p1,p2,orientation=[-60,80,0],axes=frame,labels=[x,y,z],tickmarks=[2,2,3],scaling=constrained,lightmodel=none);
print(p3);
end module:
end use:
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Figure 6.4.1 Average value of
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The volume below is given by = , exactly half the total volume. Hence, care must be taken in interpreting the meaning of the average value of a function of several variables.
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Maple Tools for Calculating Average Value
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The Student MultivariateCalculus package contains the FunctionAverage command that will construct and evaluate the average value of a function of two variables in either Cartesian or polar coordinates.
In addition, this command for functions of two variables has been implemented in two task templates, one for Cartesian coordinates and one for polar coordinates. In the Cartesian case, the task template iterates in the order ; and in the polar case, in the order .
Of course, there is always the option of implementing the relevant integrals from first principles using either the Int and int commands at top level, or the double-integral templates in the Calculus palette.
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Examples
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Example 6.4.1
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Find the average value of over , the finite region bounded by the graph of and the -axis. See Example 6.2.1 and Example 6.1.1.
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Example 6.4.2
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Find the average value of over , the finite region bounded by the graphs of and . See Example 6.2.2 and Example 6.1.2.
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Example 6.4.3
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Find the average value of over , the region bounded by the graphs of and on . See Example 6.2.3 and Example 6.1.3.
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Example 6.4.4
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Find the average value of over , the region bounded by the graphs of and on the interval .
See Example 6.2.4 and Example 6.1.4.
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Example 6.4.5
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Find the average value of over , the region bounded by the graphs of 1, , and on . See Example 6.2.7 and Example 6.1.7.
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Example 6.4.6
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Find the average value of over , the interior of the loop of the 4-leaf rose that straddles the positive -axis. See Example 5.7.1.
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Example 6.4.7
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Find the average value of over , the region that is inside the cardioid but outside the circle . See Example 5.7.2.
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Example 6.4.8
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Find the average value of over , the region that is inside the circle but outside the cardioid . See Example 5.7.3.
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Example 6.4.9
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Find the average value of over , the region that is inside the circle but outside the cardioid . See Example 5.7.4.
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Example 6.4.10
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Find the average value of over , the region that is inside the circle but outside the limaçon . See Example 5.7.5.
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