integration review problems


A number of good review-type problems involving setting up integrals and what-have-you.

1. Problem: Set up double and triple integrals for the volume inside both of the cylinders x2 + y2 = 1 and y2 + z2 = 4.

figure of the first cylinder
figure 1: the first cylinder
figure of the second cylinder
figure 2: the second cylinder
figure of the ice cream cone
figure 3: the "ice cream cone"

Answer: Note that we don't actually have to be able to see the three dimensional volume to set this up. For the triple integral, we want int int int_E dV, where E is the volume we're considering. To do this, think about projecting it into one plane and looking at int int_projection int dz dA.

What do the cylinders look like? A quick sketch is shown in figures 1 and 2 to the right.

We'll project into the xy-plane, where the projection is just x2 + y2 = 1. So the integral over the projection in the xy-plane is easily written in polar coordinates: int_0^{2pi} int_0^1 dA.

Then the limits on the innermost integral are determined by the range of z-values taken on in the volume. These are given by the second cylinder, y2 + z2 = 4. On the bottom of this, z = -sqrt(4 - y2) and on the top, z = sqrt(4 - y2).

Thus the volume integral is Volume = int_0^{2pi} int_0^1 int_{-sqrt(4-(r sin(t))^2)}^{sqrt(4-(r sin(t))^2)} r dz dr dt (after converting y into polar coordinates).

The double integral giving volume is directly related to this: we want int int_R (height) dA, where (height) is the vertical height of the volume at any point in the projected region R. This is the same as simply evaluating the innermost integral in the cylindrical coordinates triple integral that we found above. Thus we get Volume = int_0^{2pi} int_0^1 2 sqrt(4 - (r sin(t))^2) r dr dt.

2. Problem: If delta = 3x + y + z is the mass of an "ice cream cone", set up an integral in spherical, cylindrical and rectangular coordinates to find the mass. The side of the cone is z = 2 sqrt(x2 + y2) and the top is a spherical cap with radius 3.

Answer: The ice cream cone is shown in figure 3 to the right. Because we are told that the boundaries are a spherical cap and a cone, both of which are given by constant values of rho and phi, respectively, we would normally set this up in spherical coordinates. In spherical coordinates, we can immediately see that 0 <= rho <= 3 and 0 <= theta <= 2pi.

To find the range for phi, look at the figure from the side: the smallest value of phi that we want is when phi = 0 (that is, along the z-axis), and the largest value gives the edge of the cone. Because the figure looks the same from all sides, we can consider any view we like. Look at the yz-plane. There x = 0, so z = 2y. This says that the ratio of the two legs in the triangle drawn on the right side of the cone is 1:2 (y:z), and the maximum angle is phi = arctan(1/2). (Be sure that you see why this is.)

Thus, the integral in spherical coordinates (after converting the density delta into spherical coordinates) is

int_0^{2pi} int_0^3 int_0^arctan(1/2) (3 rho sin(phi) cos(theta) + rho sin(phi) sin(theta) + rho cos(phi)) (rho^2 sin(phi)) dphi drho dtheta

What about cylindrical coordinates? In this case we project down into the xy-plane to get a circle. The radius of the circle is where the cone intersects the sphere of radius 3, so we have both that z = 2 sqrt(x2 + y2) and x2 + y2 + z2 = 9. Thus x2 + y2 + 4 (x2 + y2) = 9, or 5 (x2 + y2) = 9, so that r = 3/sqrt(5).

Thus the outer two limits for the cylindrical coordinates integral are 0 <= r <= 3/sqrt(5) and 0 <= theta <= 2pi.

For the z limits, notice that at any point in the circle the bottom value of z we want is given by the cone, and the top is given by the sphere. Thus we must have 2 sqrt(x2 + y2) < = z <= sqrt(9 - (x2 + y2)), or 2 r <= z <= sqrt(9 - r2).

Thus the final integral in cylindrical coordinates is

int_0^{2pi} int_0^{3/sqrt(5)} int_{2r}^{sqrt(9-r^2)} (3 r cos(theta) + r sin(theta) + z) (r) dz dr dtheta

Finally, in rectangular coordinates, we get the integral

int_{-3/sqrt(5)}^{3/sqrt(5)} int_{-sqrt((9/5)-x^2)}^{sqrt((9/5)-x^2)} int_{2sqrt(x^2+y^2)}^{sqrt(9-(x^2+y^2))} (3x + y + z) dz dy dx

The spherical coordinates integral is nicer.

3. Problem: It the top of the ice cream cone is flat, say, at z = 3, how does your answer change?

volume of last region
figure 4: the last region

Answer: In this case it doesn't make sense to use spherical coordinates. The flat top (z = 3) is not well described in spherical coordinates, so we'd default to cylindrical coordinates. The integral will be similar to that shown above, but because we've said that z = 3, not rho = 3 the radius of the circle changes and we get

int_0^{2pi} int_0^{3/2} int_{2r}^{3} (3 r cos(theta) + r sin(theta) + z) (r) dz dr dtheta

Check that you understand how all of the limits were arrived at for this problem.

4. Problem: Sketch and convert to cylindrical coordinates the integral

int_0^2 int_0^{sqrt(4-y^2)} int_0^{2sqrt(2)-x-y} 5xy dz dx dy

Answer: If we didn't have to sketch this, we could do the conversion almost routinely. The domain in the xy-plane we can see from the outer two limits in the integral above is a quarter circle with radius 2. Thus we can convert the integral without thinking too hard: the outer two limits are 0 <= r <= 2 and 0 <= theta <= pi/2. Finally, we just have to change the z limits into polar coordinates, and we're done:

int_0^{pi/2} int_0^2 int_0^{2sqrt(2)-r cos(theta)-r sin(theta)} 5r^2 cos(theta) sin(theta) r dz dr dtheta

What does the region look like, though? The domain in the xy-plane is a quarter circle, and the top of the figure is determined by the plane z = 2sqrt(2) - x - y. A crude sketch is shown to the right above.

Last modified: Wed Nov 17 18:06:03 EST 2004
Comments to:glarose(at)umich(dot)edu
©2004 Gavin LaRose, UM Math Dept.