# 13: Multiple Integration

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In this chapter we apply techniques of integral calculus to multivariable functions. In Chapter 5 we learned how the definite integral of a single variable function gave us "area under the curve." In this chapter we will see that integration applied to a multivariable function gives us "volume under a surface." And just as we learned applications of integration beyond finding areas, we will find applications of integration in this chapter beyond finding volume.

• 13.1: Iterated Integrals and Area
Previously, we found that it was useful to differentiate functions of several variables with respect to one variable, while treating all the other variables as constants or coefficients. We can integrate functions of several variables in a similar way.
• 13.2: Double Integration and Volume
This section has extended our understanding of iterated integrals; now we see they can be used to find the signed volume under a surface.
• 13.3: Double Integration with Polar Coordinates
We have used iterated integrals to find areas of plane regions and volumes under surfaces. Just as a single integral can be used to compute much more than "area under the curve,'' iterated integrals can be used to compute much more than we have thus far seen. The next two sections show two, among many, applications of iterated integrals.
• 13.4: Center of Mass
We have used iterated integrals to find areas of plane regions and signed volumes under surfaces. A brief recap of these uses will be useful in this section as we apply iterated integrals to compute the mass and center of mass of planar regions. This section has shown us another use for iterated integrals beyond finding area or signed volume under the curve. While there are many uses for iterated integrals, we give one more application in the following section: computing surface area.
• 13.5: Surface Area
The natural extension of the concept of "arc length over an interval'' to surfaces is "surface area over a region.''
• 13.6: Volume Between Surfaces and Triple Integration
• 13.E: Applications of Multiple Integration (Exercises)