
# 3.E: Multiple Integrals (Exercises)

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## 3.1: Double Integrals

#### A

For Exercises 1-4, find the volume under the surface $$z = f (x, y)$$ over the rectangle $$R$$.

3.1.1. $$f (x, y) = 4x y,\, R = [0,1]×[0,1]$$

3.1.2. $$f (x, y) = e^{ x+y} ,\, R = [0,1]×[−1,1]$$

3.1.3. $$f (x, y) = x^ 3 + y^ 2 ,\, R = [0,1]×[0,1]$$

3.1.4. $$f (x, y) = x^ 4 + x y+ y^ 3 ,\, R = [1,2]×[0,2]$$

For Exercises 5-12, evaluate the given double integral.

3.1.5. $$\int_0^1 \int_1^2 (1− y)x^ 2 \,dx \,d y$$

3.1.6. $$\int_0^1 \int_0^2 x(x+ y)\,dx \,d y$$

3.1.7. $$\int_0^2 \int_0^1 (x+2)\,dx \,d y$$

3.1.8. $$\int_{−1}^2 \int_{−1}^1 x(x y+\sin x)\,dx\, d y$$

3.1.9. $$\int_0^{\pi /2} \int_0^1 x y\cos (x^ 2 y)\,dx\, d y$$

3.1.10. $$\int_0^{\pi} \int_0^{π/2} \sin x \cos (y−π) \,dx\, d y$$

3.1.11. $$\int_0^2 \int_1^4 x y \,dx\, d y$$

3.1.12. $$\int_{-1}^1 \int_{-1}^2 1\,dx\, d y$$

3.1.13. Let $$M$$ be a constant. Show that $$\int_c^d \int_a^b M\, dx \,d y = M(d − c)(b − a).$$

## 3.2: Double Integrals Over a General Region

#### A

For Exercises 1-6, evaluate the given double integral.

3.2.1. $$\int_0^1 \int_{\sqrt{ x}}^1 24x^ 2 y \,d y\, dx$$

3.2.2. $$\int_0^π \int_0^y \sin x \,dx\, d y$$

3.2.3. $$\int_1^2 \int_0^{\ln x} 4x \,d y\, dx$$

3.2.4. $$\int_0^2 \int_0^{2y} e^ {y^ 2} \,dx \,d y$$

3.2.5. $$\int_0^{π/2} \int_0^y \cos x \sin y \,dx \,d y$$

3.2.6. $$\int_0^{∞} \int_0^{∞} x ye^{−(x^ 2+y^ 2 )}\, dx \,d y$$

3.2.7. $$\int_0^2 \int_0^y 1\,dx \,d y$$

3.2.8. $$\int_0^1 \int_0^{x^ 2} 2\,d y\, dx$$

3.2.9. Find the volume $$V$$ of the solid bounded by the three coordinate planes and the plane $$x+ y+ z = 1$$.

3.2.10. Find the volume $$V$$ of the solid bounded by the three coordinate planes and the plane $$3x+2y+5z = 6$$.

#### B

3.2.11. Explain why the double integral $$\iint\limits_R 1\,d A$$ gives the area of the region $$R$$. For simplicity, you can assume that $$R$$ is a region of the type shown in Figure 3.2.1(a).

#### C

3.2.12. Prove that the volume of a tetrahedron with mutually perpendicular adjacent sides of lengths $$a,\, b, \text{ and }c$$, as in Figure 3.2.6, is $$\frac{abc}{ 6}$$ . (Hint: Mimic Example 3.5, and recall from Section 1.5 how three noncollinear points determine a plane.)

Figure 3.2.6

3.2.13. Show how Exercise 12 can be used to solve Exercise 10.

## 3.3: Triple Integrals

#### A

For Exercises 1-8, evaluate the given triple integral.

3.3.1. $$\int_0^3 \int_0^2 \int_0^1 x yz \,dx\, d y\, dz$$

3.3.2. $$\int_0^1 \int_0^x \int_0^y x yz \,dz\, d y\, dx$$

3.3.3. $$\int_0^π \int_0^x \int_0^{x y} x^ 2 \sin z \,dz\, d y\, dx$$

3.3.4. $$\int_0^1 \int_0^z \int_0^y ze^{ y^ 2} \,dx\, d y\, dz$$

3.3.5. $$\int_1^e \int_0^y \int_0^{1/y} x^ 2 z \,dx \,dz \,d y$$

3.3.6. $$\int_1^2 \int_0^{y^ 2} \int_0^{z^ 2} yz \,dx \,dz \,d y$$

3.3.7. $$\int_1^2 \int_2^4 \int_0^3 1\,dx \,d y\, dz$$

3.3.8. $$\int_0^1 \int_0^{1−x} \int_0^{1−x−y} 1\,dz\, d y\, dx$$

3.3.9. Let $$M$$ be a constant. Show that $$\int_{z_1}^{z_2} \int_{y_1}^{y_2} \int_{x_1}^{x_2} M\, dx\, d y\, dz = M(z_2 − z_1)(y_2 − y_1)(x_2 − x_1)$$.

#### B

3.3.10. Find the volume $$V$$ of the solid $$S$$ bounded by the three coordinate planes, bounded above by the plane $$x+ y+ z = 2$$, and bounded below by the plane $$z = x+ y$$.

#### C

3.3.11. Show that $$\int_a^b \int_a^z \int_a^y f (x)\,dx \,d y \,dz = \int_a^b \frac{(b−x)^ 2}{ 2} f (x)\,dx$$. (Hint: Think of how changing the order of integration in the triple integral changes the limits of integration.)

## 3.4: Numerical Approximation of Multiple Integrals

#### C

3.4.1. Write a program that uses the Monte Carlo method to approximate the double integral $$\iint\limits_R e^{ x y}\, d A$$, where $$R = [0,1]×[0,1]$$. Show the program output for $$N = 10,\, 100,\, 1000,\, 10000,\, 100000 \text{ and }1000000$$ random points.

3.4.2. Write a program that uses the Monte Carlo method to approximate the triple integral \iiint\limits_S e^{ x yz}\, dV\), where $$S = [0,1] × [0,1] × [0,1]$$. Show the program output for $$N = 10,\, 100,\, 1000,\, 10000,\, 100000 \text{ and }1000000$$ random points.

3.4.3. Repeat Exercise 1 with the region $$R = {(x, y) : −1 ≤ x ≤ 1,\, 0 ≤ y ≤ x^ 2 }$$.

3.4.4. Repeat Exercise 2 with the solid $$S = {(x, y, z) : 0 ≤ x ≤ 1,\, 0 ≤ y ≤ 1, \,0 ≤ z ≤ 1− x− y}$$.

3.4.5. Use the Monte Carlo method to approximate the volume of a sphere of radius 1.

3.4.6. Use the Monte Carlo method to approximate the volume of the ellipsoid $$\frac{x^ 2}{ 9} + \frac{y^ 2}{ 4} + \frac{z^ 2}{ 1} = 1$$.

## 3.5: Change of Variables in Multiple Integrals

#### A

3.5.1. Find the volume $$V$$ inside the paraboloid $$z = x^ 2 + y^ 2 \text{ for }0 ≤ z ≤ 4$$.

3.5.2. Find the volume $$V$$ inside the cone $$z = \sqrt{ x^ 2 + y^ 2}$$ for $$0 ≤ z ≤ 3$$.

#### B

3.5.3. Find the volume $$V$$ of the solid inside both $$x^ 2 + y^ 2 + z^ 2 = 4$$ and $$x^ 2 + y^ 2 = 1$$.

3.5.4. Find the volume $$V$$ inside both the sphere $$x^ 2 + y^ 2 + z^ 2 = 1$$ and the cone $$z = \sqrt{ x^ 2 + y^ 2}$$.

3.5.5. Prove Equation (3.25).

3.5.6. Prove Equation (3.26).

3.5.7. Evaluate $$\iiint\limits_R \sin \left ( \frac{x+y}{ 2} \right ) \cos \left ( \frac{x−y}{ 2} \right ) \,d A$$, where $$R$$ is the triangle with vertices $$(0,0),\, (2,0) \text{ and }(1,1)$$. (Hint: Use the change of variables $$u = (x+ y)/2,\, v = (x− y)/2.$$)

3.5.8. Find the volume of the solid bounded by $$z = x^ 2 + y^ 2 \text{ and }z^ 2 = 4(x^ 2 + y^ 2 )$$.

3.5.9. Find the volume inside the elliptic cylinder $$\frac{x^ 2}{ a^ 2} + \frac{y^ 2}{ b^ 2} = 1 \text{ for } 0 ≤ z ≤ 2$$.

#### C

3.5.10. Show that the volume inside the ellipsoid $$\frac{x^ 2}{ a^ 2} + \frac{y^ 2}{ b^ 2} + \frac{z^ 2}{ c^ 2} = 1 \text{ is }\frac{4πabc}{ 3}$$ . (Hint: Use the change of variables $$x = au,\, y = bv,\, z = cw$$, then consider Example 3.12.)

3.5.11. Show that the Beta function, defined by

$B(x, y) = \int_0^1 t^{ x−1} (1− t)^{ y−1} dt ,\text{ for }x > 0,\, y > 0,$

satisfies the relation $$B(y, x) = B(x, y) \text{ for }x > 0,\, y > 0.$$

3.5.12. Using the substitution $$t = u/(u +1)$$, show that the Beta function can be written as

$B(x, y) = \int_0^∞ \frac{u^{ x−1}}{ (u +1)^{x+y}}\, du ,\text{ for }x > 0,\, y > 0.$

## 3.6: Application: Center of Mass

#### A

For Exercises 1-5, find the center of mass of the region $$R$$ with the given density function $$δ(x, y)$$.

3.6.1. $$R = {(x, y) : 0 ≤ x ≤ 2,\, 0 ≤ y ≤ 4 },\, δ(x, y) = 2y$$

3.6.2. $$R = {(x, y) : 0 ≤ x ≤ 1,\, 0 ≤ y ≤ x 2 },\, δ(x, y) = x+ y$$

3.6.3. $$R = {(x, y) : y ≥ 0,\, x^ 2 + y^ 2 ≤ a^ 2 },\, δ(x, y) = 1$$

3.6.4. $$R = {(x, y) : y ≥ 0,\, x ≥ 0,\, 1 ≤ x^ 2 + y^ 2 ≤ 4 },\, δ(x, y) = \sqrt{ x^ 2 + y^ 2}$$

3.6.5. $$R = {(x, y) : y ≥ 0,\, x^ 2 + y^ 2 ≤ 1 },\, δ(x, y) = y$$

#### B

For Exercises 6-10, find the center of mass of the solid $$S$$ with the given density function $$δ(x, y, z)$$.

3.6.6. $$S = {(x, y, z) : 0 ≤ x ≤ 1,\, 0 ≤ y ≤ 1,\, 0 ≤ z ≤ 1 },\, δ(x, y, z) = x yz$$

3.6.7. $$S = {(x, y, z) : z ≥ 0,\, x^ 2 + y^ 2 + z^ 2 ≤ a^ 2 },\, δ(x, y, z) = x^ 2 + y^ 2 + z^ 2$$

3.6.8. $$S = {(x, y, z) : x ≥ 0,\, y ≥ 0,\, z ≥ 0,\, x^ 2 + y^ 2 + z^ 2 ≤ a^ 2 },\, δ(x, y, z) = 1$$

3.6.9. $$S = {(x, y, z) : 0 ≤ x ≤ 1,\, 0 ≤ y ≤ 1,\, 0 ≤ z ≤ 1 },\, δ(x, y, z) = x^ 2 + y^ 2 + z^ 2$$

3.6.10. $$S = {(x, y, z) : 0 ≤ x ≤ 1,\, 0 ≤ y ≤ 1,\, 0 ≤ z ≤ 1− x− y},\, δ(x, y, z) = 1$$

## 3.7: Application: Probability and Expected Value

#### B

3.7.1. Evaluate the integral $$\int_{−\infty}^{\infty} e^{ −x^ 2}\, dx$$ using anything you have learned so far.

3.7.2. For $$σ > 0 \text{ and }µ > 0$$, evaluate $$\int_{\infty}^{−\infty} \frac{1}{ σ \sqrt{ 2π}} e^{ −(x−µ)^ 2 /2σ^ 2} dx$$.

3.7.3. Show that $$EY = \frac{n}{ n+1}$$ in Example 3.18

#### C

3.7.4. Write a computer program (in the language of your choice) that verifies the results in Example 3.18 for the case $$n = 3$$ by taking large numbers of samples.

3.7.5. Repeat Exercise 4 for the case when $$n = 4$$.

3.7.6. For continuous random variables $$X, Y \text{ with joint p.d.f. }f (x, y)$$, define the second moments $$E(X^ 2 ) \text{ and }E(Y^ 2 )$$ by

$E(X^ 2 ) = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} x^ 2 f (x, y)\,dx\, d y \text{ and }E(Y^ 2 ) = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} y^ 2 f (x, y)\,dx \,d y ,$

and the variances Var$$(X)$$ and Var$$(Y)$$ by

$\text{Var}(X) = E(X^ 2 )−(EX)^ 2 \text{ and Var}(Y) = E(Y^ 2 )−(EY)^ 2 .$

Find Var$$(X)$$ and Var$$(Y)$$ for $$X$$ and $$Y$$ as in Example 3.18.

3.7.7. Continuing Exercise 6, the correlation $$ρ \text{ between }X \text{ and }Y$$ is defined as

$ρ = \frac{E(XY)−(EX)(EY)}{ \sqrt{ \text{Var}(X)\text{Var}(Y)}} ,$

where $$E(XY) = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} x y \,f (x, y)\,dx\, d y$$. Find $$ρ$$ for $$X \text{ and }Y$$ as in Example 3.18.
(Note: The quantity $$E(XY)−(EX)(EY)$$ is called the covariance of $$X$$ and $$Y$$.)

3.7.8. In Example 3.17 would the answer change if the interval $$(0,100)$$ is used instead of $$(0,1)$$? Explain.