
# 15: Multiple Integration


In this chapter we extend the concept of a definite integral of a single variable to double and triple integrals of functions of two and three variables, respectively. We examine applications involving integration to compute volumes, masses, and centroids of more general regions. We will also see how the use of other coordinate systems (such as polar, cylindrical, and spherical coordinates) makes it simpler to compute multiple integrals over some types of regions and functions. In the preceding chapter, we discussed differential calculus with multiple independent variables. Now we examine integral calculus in multiple dimensions. Just as a partial derivative allows us to differentiate a function with respect to one variable while holding the other variables constant, we will see that an iterated integral allows us to integrate a function with respect to one variable while holding the other variables constant.

• 15.1: Prelude to Multiple Integration
In the preceding chapter, we discussed differential calculus with multiple independent variables. Now we examine integral calculus in multiple dimensions. Just as a partial derivative allows us to differentiate a function with respect to one variable while holding the other variables constant, we will see that an iterated integral allows us to integrate a function with respect to one variable while holding the other variables constant.
• 15.2: Double Integrals over Rectangular Regions
In this section we investigate double integrals and show how we can use them to find the volume of a solid over a rectangular region in the xyxy-plane. Many of the properties of double integrals are similar to those we have already discussed for single integrals.
• 15.3: Double Integrals over General Regions
In this section we consider double integrals of functions defined over a general bounded region D on the plane. Most of the previous results hold in this situation as well, but some techniques need to be extended to cover this more general case.
• 15.4: Double Integrals in Polar Coordinates
Double integrals are sometimes much easier to evaluate if we change rectangular coordinates to polar coordinates. However, before we describe how to make this change, we need to establish the concept of a double integral in a polar rectangular region.
• 15.5: Triple Integrals
In Double Integrals over Rectangular Regions, we discussed the double integral of a function f(x,y) of two variables over a rectangular region in the plane. In this section we define the triple integral of a function f(x,y,z) of three variables over a rectangular solid box in space, R³. Later in this section we extend the definition to more general regions in R³.
• 15.6: Triple Integrals in Cylindrical and Spherical Coordinates
In this section we convert triple integrals in rectangular coordinates into a triple integral in either cylindrical or spherical coordinates.
• 15.7: Calculating Centers of Mass and Moments of Inertia
In this section we develop computational techniques for finding the center of mass and moments of inertia of several types of physical objects, using double integrals for a lamina (flat plate) and triple integrals for a three-dimensional object with variable density. The density is usually considered to be a constant number when the lamina or the object is homogeneous; that is, the object has uniform density.
• 15.8: Change of Variables in Multiple Integrals
When solving integration problems, we make appropriate substitutions to obtain an integral that becomes much simpler than the original integral. We also used this idea when we transformed double integrals in rectangular coordinates to polar coordinates and transformed triple integrals in rectangular coordinates to cylindrical or spherical coordinates to make the computations simpler.
• 15.E: Multiple Integration (Exercises)
These are homework exercises to accompany Chapter 15 of OpenStax's "Calculus" Textmap.

Thumbnail: Double integral as volume under a surface $$z = 10 − x^2 − y^2/8$$. The rectangular region at the bottom of the body is the domain of integration, while the surface is the graph of the two-variable function to be integrated. Image used with permission (Public Domain; Oleg Alexandrov).