1. SECTION 12.5
TRIPLE INTEGRALS

2. TRIPLE INTEGRALS
Just as we defined single integrals for functions of one variable and double integrals for functions of two variables, so we can define triple integrals for functions of three variables.
Let’s first deal with the simplest case where f is defined on a rectangular box:
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3. The first step is to divide B into sub-boxes—by dividing:
TRIPLE INTEGRALS
The first step is to divide B into sub-boxes—by dividing:
The interval [a, b] into l subintervals [xi–1, xi] of equal width Δxi = xi – xi–1.
[c, d] into m subintervals of width Δyj = yj – yj–1.
[r, s] into n subintervals of width Δzk = zk – zk–1.
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4. which are shown in Figure 1.
TRIPLE INTEGRALS
The planes through the endpoints of these subintervals parallel to the coordinate planes divide the box B into lmn sub-boxes
which are shown in Figure 1.
Each sub-box Bijk has volume
ΔVijk = ΔxiΔyjΔzk
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5. TRIPLE INTEGRALS
Then, we form the triple Riemann sum where the sample point is in Bijk.
By analogy with the definition of a double integral (Definition 5 in Section 12.1), we define the triple integral as the limit of the triple Riemann sums in Equation 2.
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6. The triple integral of f over the box B is:
Definition 3
The triple integral of f over the box B is:
if this limit exists.
Again, the triple integral always exists if f is continuous.
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7. We can choose the sample point to be any point in the sub-box.
TRIPLE INTEGRALS
We can choose the sample point to be any point in the sub-box.
However, if we choose it to be the point (xi, yj, zk) we get a simpler-looking expression:
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8. TRIPLE INTEGRALS
Just as for double integrals, the practical method for evaluating triple integrals is to express them as iterated integrals, as follows.
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9. FUBINI’S THEOREM FOR TRIPLE INTEGRALS
If f is continuous on the rectangular box
B = [a, b] × [c, d] × [r, s],
then
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10. FUBINI’S THEOREM FOR TRIPLE INTEGRALS
The iterated integral on the right side of Fubini’s Theorem means that we integrate in the following order:
With respect to x (keeping y and z fixed)
With respect to y (keeping z fixed)
With respect to z
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11. FUBINI’S THEOREM FOR TRIPLE INTEGRALS
There are five other possible orders in which we can integrate, all of which give the same value.
For instance, if we integrate with respect to y, then z, and then x, we have:
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12. Evaluate the triple integral where B is the rectangular box
Example 1
Evaluate the triple integral where B is the rectangular box
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13. We could use any of the six possible orders of integration.
Example 1 SOLUTION
We could use any of the six possible orders of integration.
If we choose to integrate with respect to x, then y, and then z, we obtain
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14. INTEGRAL OVER BOUNDED REGION
Now, we define the triple integral over a general bounded region E in three-dimensional space (a solid) by much the same procedure that we used for double integrals.
See Definition 2 in Section 12.2.
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15. INTEGRAL OVER BOUNDED REGION
We enclose E in a box B of the type given by Equation 1.
Then, we define a function F so that it agrees with f on E but is 0 for points in B that are outside E.
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16. INTEGRAL OVER BOUNDED REGION
By definition,
This integral exists if f is continuous and the boundary of E is “reasonably smooth.”
The triple integral has essentially the same properties as the double integral (Properties 6–9 in Section 12.2).
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17. INTEGRAL OVER BOUNDED REGION
We restrict our attention to:
Continuous functions f
Certain simple types of regions
A solid region E is said to be of type 1 if it lies between the graphs of two continuous functions of x and y.
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18. TYPE 1 REGION
That is, where D is the projection of E onto the xy-plane as shown in Figure 2.
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19. TYPE 1 REGIONS Notice that:
The upper boundary of the solid E is the surface with equation z = u2(x, y).
The lower boundary is the surface z = u1(x, y).
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20. TYPE 1 REGIONS
By the same sort of argument that led to Formula 3 in Section 12.2, it can be shown that, if E is a type 1 region given by Equation 5, then
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21. TYPE 1 REGIONS
The meaning of the inner integral on the right side of Equation 6 is that x and y are held fixed.
Therefore,
u1(x, y) and u2(x, y) are regarded as constants.
f(x, y, z) is integrated with respect to z.
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22. TYPE 1 REGIONS
In particular, if the projection D of E onto the xy-plane is a type I plane region (as in Figure 3), then
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23. Thus, Equation 6 becomes:
TYPE 1 REGIONS
Thus, Equation 6 becomes:
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24. If, instead, D is a type II plane region (as in Figure 4), then
TYPE 1 REGIONS
If, instead, D is a type II plane region (as in Figure 4), then
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25. Then, Equation 6 becomes:
TYPE 1 REGIONS
Then, Equation 6 becomes:
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26. Evaluate where E is the solid tetrahedron bounded by the four planes
Example 2
Evaluate where E is the solid tetrahedron bounded by the four planes
x = 0, y = 0, z = 0, x + y + z = 1
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27. When we set up a triple integral, it’s wise to draw two diagrams:
Example 2 SOLUTION
When we set up a triple integral, it’s wise to draw two diagrams:
The solid region E (See Figure 5)
Its projection D on the xy-plane (See Figure 6)
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28. Example 2 SOLUTION
The lower boundary of the tetrahedron is the plane z = 0 and the upper boundary is the plane x + y + z = 1 (or z = 1 – x – y).
So, we use u1(x, y) = 0 and u2(x, y) = 1 – x – y in Formula 7.
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29. Example 2 SOLUTION
Notice that the planes x + y + z = 1 and z = 0 intersect in the line x + y = 1 (or y = 1 – x) in the xy-plane.
So, the projection of E is the triangular region shown in Figure 6, and we have the following equation.
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30. Example 2 SOLUTION
This description of E as a type 1 region enables us to evaluate the integral as follows.
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31. TYPE 2 REGION
A solid region E is of type 2 if it is of the form where D is the projection of E onto the yz-plane.
See Figure 7.
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32. The back surface is x = u1(y, z). The front surface is x = u2(y, z).
TYPE 2 REGION
The back surface is x = u1(y, z).
The front surface is x = u2(y, z).
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33. TYPE 2 REGION
Thus, we have:
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34. Finally, a type 3 region is of the form where:
D is the projection of E onto the xz-plane.
y = u1(x, z) is the left surface.
y = u2(x, z) is the right surface.
See Figure 8.
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35. For this type of region, we have:
TYPE 3 REGION
For this type of region, we have:
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36. TYPE 2 & 3 REGIONS
In each of Equations 10 and 11, there may be two possible expressions for the integral depending on:
Whether D is a type I or type II plane region (and corresponding to Equations 7 and 8).
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37. Example 3
Evaluate where E is the region bounded by the paraboloid y = x2 + z2 and the plane y = 4.
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38. The solid E is shown in Figure 9.
Example 3 SOLUTION
The solid E is shown in Figure 9.
If we regard it as a type 1 region, then we need to consider its projection D1 onto the xy-plane.
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39. That is the parabolic region shown in Figure 10.
Example 3 SOLUTION
That is the parabolic region shown in Figure 10.
The trace of y = x2 + z2 in the plane z = 0 is the parabola y = x2
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40. Example 3 SOLUTION From y = x2 + z2, we obtain:
So, the lower boundary surface of E is:
The upper surface is:
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41. Therefore, the description of E as a type 1 region is:
Example 3 SOLUTION
Therefore, the description of E as a type 1 region is:
Thus, we obtain:
Although this expression is correct, it is extremely difficult to evaluate.
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42. So, let’s instead consider E as a type 3 region.
Example 3 SOLUTION
So, let’s instead consider E as a type 3 region.
As such, its projection D3 onto the xz-plane is the disk x2 + z2 ≤ 4 shown in Figure 11.
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43. Then, the left boundary of E is the paraboloid y = x2 + z2.
Example 3 SOLUTION
Then, the left boundary of E is the paraboloid y = x2 + z2.
The right boundary is the plane y = 4.
So, taking u1(x, z) = x2 + z2 and u2(x, z) = 4 in Equation 11, we have:
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44. This integral could be written as:
Example 3 SOLUTION
This integral could be written as:
However, it’s easier to convert to polar coordinates in the xz-plane:
x = r cos , z = r sin 
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45. Example 3 SOLUTION
That gives:
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46. Example 4
Express the iterated integral as a triple integral and then rewrite it as iterated integral in a different order, integrating first with respect to x, then z, and then y.
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47. Example 4 SOLUTION
We can write where
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48. Example 4 SOLUTION
This description of E enable us to write projections onto the three coordinate planes as follows：
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49. Example 4 SOLUTION
From the resulting of the projections in Figure 12 we sketch the solid E in Figure 13.
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50. Example 4 SOLUTION
We see that it is the solid enclosed by the planes and the parabolic cylinder
If we integrate first with respect to x, then z, and then y, we use an alternate description of E：
Thus
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51. APPLICATIONS OF TRIPLE INTEGRALS
Recall that:
If f(x) ≥ 0, then the single integral represents the area under the curve y = f(x) from a to b.
If f(x, y) ≥ 0, then the double integral represents the volume under the surface z = f(x, y) and above D.
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52. APPLICATIONS OF TRIPLE INTEGRALS
The corresponding interpretation of a triple integral , where f(x, y, z) ≥ 0, is not very useful.
It would be the “hypervolume” of a four-dimensional (4-D) object.
Of course, that is very difficult to visualize.
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53. APPLICATIONS OF TRIPLE INTEGRALS
Remember that E is just the domain of the function f.
The graph of f lies in 4-D space.
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54. APPLICATIONS OF TRIPLE INTEGRALS
Nonetheless, the triple integral can be interpreted in different ways in different physical situations.
This depends on the physical interpretations of x, y, z and f(x, y, z).
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55. APPLNS. OF TRIPLE INTEGRALS
Let’s begin with the special case where f(x, y, z) = 1 for all points in E.
Then, the triple integral does represent the volume of E:
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56. APPLNS. OF TRIPLE INTEGRALS
For example, you can see this in the case of a type 1 region by putting f(x, y, z) = 1 in Formula 6:
From Section 12.2, we know this represents the volume that lies between the surfaces z = u1(x, y) and z = u2(x, y)
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57. Example 5
Use a triple integral to find the volume of the tetrahedron T bounded by the planes x + 2y + z = x = 2y x = z = 0
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58. Example 5 SOLUTION
The tetrahedron T and its projection D on the xy-plane are shown in Figure 14 and 15.
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59. The lower boundary of T is the plane z = 0.
Example 5 SOLUTION
The lower boundary of T is the plane z = 0.
The upper boundary is the plane x + 2y + z = 2, that is, z = 2 – x – 2y
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60. Example 5 SOLUTION So, we have:
This is obtained by the same calculation as in Example 4 in Section 12.2
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61. Example 5 SOLUTION
Notice that it is not necessary to use triple integrals to compute volumes.
They simply give an alternative method for setting up the calculation.
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62. APPLICATIONS
All the applications of double integrals in Section 12.4 can be immediately extended to triple integrals.
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63. in units of mass per unit volume, at any given point (x, y, z).
APPLICATIONS
For example, suppose the density function of a solid object that occupies the region E is:
r(x, y, z)
in units of mass per unit volume, at any given point (x, y, z).
Then, its mass is:
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64. Its moments about the three coordinate planes are:
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65. The center of mass is located at the point , where:
If the density is constant, the center of mass of the solid is called the centroid of E.
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66. The moments of inertia about the three coordinate axes are:
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67. TOTAL ELECTRIC CHARGE
As in Section 12.4, the total electric charge on a solid object occupying a region E and having charge density s(x, y, z) is:
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68. Example 6
Find the center of mass of a solid of constant density that is bounded by the parabolic cylinder x = y2 and the planes x = z, z = 0, and x = 1.
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69. Example 6 SOLUTION
The solid E and its projection onto the xy-plane are shown in Figure 16.
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70. The lower and upper surfaces of E are the planes z = 0 and z = x.
Example 6 SOLUTION
The lower and upper surfaces of E are the planes z = 0 and z = x.
So, we describe E as a type 1 region:
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71. Then, if the density is r(x, y, z) = r, the mass is:
Example 6 SOLUTION
Then, if the density is r(x, y, z) = r, the mass is:
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72. The other moments are calculated as follows.
Example 6 SOLUTION
Due to the symmetry of E and r about the xz-plane, we can immediately say that Mxz = 0, and therefore
The other moments are calculated as follows.
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73. Example 6 SOLUTION
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74. Example 6 SOLUTION
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75. Therefore, the center of mass is:
Example 6 SOLUTION
Therefore, the center of mass is:
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