388 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

Section 5.8 Hyperbolic Functions

• Develop properties of hyperbolic functions.• Differentiate and integrate hyperbolic functions.• Develop properties of inverse hyperbolic functions.• Differentiate and integrate functions involving inverse hyperbolic functions.

Hyperbolic Functions

In this section you will look briefly at a special class of exponential functions calledhyperbolic functions. The name hyperbolic function arose from comparison of thearea of a semicircular region, as shown in Figure 5.35, with the area of a region undera hyperbola, as shown in Figure 5.36. The integral for the semicircular region involvesan inverse trigonometric (circular) function:

The integral for the hyperbolic region involves an inverse hyperbolic function:

This is only one of many ways in which the hyperbolic functions are similar to thetrigonometric functions.

NOTE is read as “the hyperbolic sine of as “the hyperbolic cosine of andso on.

x,”cosh xx,”sinh x

�1

�1

�1 � x2 dx �12�x�1 � x2 � sinh�1

x �1

�1� 2.296.

�1

�1

�1 � x2 dx �12�x�1 � x2 � arcsin x �

1

�1�

�

2� 1.571.

x−1 1

2

y = 1 − x2

y

Circle: Figure 5.35

x2 � y2 � 1

x−1 1

2y = 1 + x2

y

Hyperbola: Figure 5.36

�x2 � y2 � 1

FOR FURTHER INFORMATION Formore information on the developmentof hyperbolic functions, see the article“An Introduction to HyperbolicFunctions in Elementary Calculus”by Jerome Rosenthal in MathematicsTeacher. To view this article, go to thewebsite www.matharticles.com.

Definitions of the Hyperbolic Functions

x � 0coth x �1

tanh x,tanh x �

sinh xcosh x

sech x �1

cosh xcosh x �

ex � e�x

2

x � 0csch x �1

sinh x,sinh x �

ex � e�x

2

JOHANN HEINRICH LAMBERT (1728–1777)

The first person to publish a comprehensivestudy on hyperbolic functions was JohannHeinrich Lambert, a Swiss-German mathe-matician and colleague of Euler.

Am

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of P

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SECTION 5.8 Hyperbolic Functions 389

The graphs of the six hyperbolic functions and their domains and ranges areshown in Figure 5.37. Note that the graph of can be obtained by addition ofordinates using the exponential functions and Likewise,the graph of can be obtained by addition of ordinates using the exponentialfunctions and

Many of the trigonometric identities have corresponding hyperbolic identities.For instance,

and

� sinh 2x.

�e2x � e�2x

2

2 sinh x cosh x � 2�ex � e�x

2 ��ex � e�x

2 �

� 1

�44

�e2x � 2 � e�2x

4�

e2x � 2 � e�2x

4

cosh2 x � sinh2 x � �ex � e�x

2 �2

� �ex � e�x

2 �2

h�x �12e�x.f �x �

12ex

cosh xg�x � �

12e�x.f�x �

12ex

sinh x

y = sinh xf(x) = ex

2

g(x) = −e−x

2

2

2

1

−1−2

−2

−1

1x

y

Domain: Range: ���, �

���, �

f(x) = ex

2

y = cosh x

h(x) = e−x

2

2

2−2

−2

−1

−1 1x

y

Domain: Range: 1, �

���, �

2

2−1−2

−2

−1

1

1

x

y = tanh x

y

Domain: Range: ��1, 1

���, �

y = csch x = 1sinh x2

2

1

−1

−1 1x

y

Domain: Range: Figure 5.37

���, 0 � �0, ����, 0 � �0, �

2

2−1

−1

−2

−2 1x

y = sech x = 1cosh x

y

Domain: Range: �0, 1�

���, �

2−1−2

−1

1

1

x

y = coth x = 1tanh x

y

Domain: Range: ���, �1 � �1, �

���, 0 � �0, �

FOR FURTHER INFORMATION Tounderstand geometrically the relation-ship between the hyperbolic andexponential functions, see the article “A Short Proof Linking the Hyperbolicand Exponential Functions” by MichaelJ. Seery in The AMATYC Review.

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390 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

Differentiation and Integration of Hyperbolic Functions

Because the hyperbolic functions are written in terms of and you can easilyderive rules for their derivatives. The following theorem lists these derivatives with thecorresponding integration rules.

Proof

In Exercises 98 and 102, you are asked to prove some of the other differentiation rules.

� sech2 x

�1

cosh2 x

�cosh x�cosh x � sinh x�sinh x

cosh2 x

ddx

tanh x� �ddx�

sinh xcosh x�

�ex � e�x

2� cosh x

ddx

sinh x� �ddx�

ex � e�x

2 �

e�x,ex

Hyperbolic Identities

cosh 2x � cosh2 x � sinh2 xsinh 2x � 2 sinh x cosh x

cosh2 x �1 � cosh 2x

2sinh2 x �

�1 � cosh 2x2

cosh�x � y � cosh x cosh y � sinh x sinh y

cosh�x � y � cosh x cosh y � sinh x sinh ycoth2 x � csch2 x � 1

sinh�x � y � sinh x cosh y � cosh x sinh ytanh2 x � sech2 x � 1

sinh�x � y � sinh x cosh y � cosh x sinh ycosh2 x � sinh2 x � 1

THEOREM 5.18 Derivatives and Integrals of Hyperbolic Functions

Let be a differentiable function of

�csch u coth u du � �csch u � Cddx

csch u� � ��csch u coth uu�

�sech u tanh u du � �sech u � Cddx

sech u� � ��sech u tanh uu�

�csch2 u du � �coth u � Cddx

coth u� � ��csch2 uu�

�sech2 u du � tanh u � Cddx

tanh u� � �sech2 uu�

�sinh u du � cosh u � Cddx

cosh u� � �sinh uu�

�cosh u du � sinh u � Cddx

sinh u� � �cosh uu�

x.u

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SECTION 5.8 Hyperbolic Functions 391

EXAMPLE 1 Differentiation of Hyperbolic Functions

a. b.

c.

EXAMPLE 2 Finding Relative Extrema

Find the relative extrema of

Solution Begin by setting the first derivative of equal to 0.

So, the critical numbers are and Using the Second Derivative Test, youcan verify that the point yields a relative maximum and the point yields a relative minimum, as shown in Figure 5.38. Try using a graphing utility toconfirm this result. If your graphing utility does not have hyperbolic functions, youcan use exponential functions as follows.

When a uniform flexible cable, such as a telephone wire, is suspended from twopoints, it takes the shape of a catenary, as discussed in Example 3.

EXAMPLE 3 Hanging Power Cables

Power cables are suspended between two towers, forming the catenary shown inFigure 5.39. The equation for this catenary is

The distance between the two towers is Find the slope of the catenary at the pointwhere the cable meets the right-hand tower.

Solution Differentiating produces

At the point the slope (from the left) is given by m � sinh ba

.�b, a cosh�b�a,

y� � a�1a� sinh

xa

� sinh xa

.

2b.

y � a cosh xa

.

� 12�xex � xe�x � 2ex

� 12�xex � xe�x � ex � e�x � ex � e�x

f�x � �x � 1�12�ex � e�x �

12�ex � e�x

�1, �sinh 1�0, �1x � 0.x � 1

�x � 1 sinh x � 0

f��x � �x � 1 sinh x � cosh x � cosh x � 0

f

f�x � �x � 1 cosh x � sinh x.

ddx

x sinh x � cosh x� � x cosh x � sinh x � sinh x � x cosh x

ddx

ln�cosh x� �sinh xcosh x

� tanh xddx

sinh�x2 � 3� � 2x cosh�x2 � 3

x

y

b−b

y = a cosh xa

a

CatenaryFigure 5.39

1

31

−2

−1−2

−3

y

x

(0, −1)

(1, −sinh 1)

f (x) = (x − 1) cosh x − sinh x

so is a relativemaximum. so isa relative minimum.Figure 5.38

�1, � sinh 1f��1 > 0,�0, �1f��0 < 0,

FOR FURTHER INFORMATION In Example 3, the cable is a catenary between twosupports at the same height. To learn about the shape of a cable hanging betweensupports of different heights, see the article “Reexamining the Catenary” by PaulCella in The College Mathematics Journal. To view this article, go to the websitewww.matharticles.com.

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392 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

EXAMPLE 4 Integrating a Hyperbolic Function

Find

Solution

Inverse Hyperbolic Functions

Unlike trigonometric functions, hyperbolic functions are not periodic. In fact, bylooking back at Figure 5.37, you can see that four of the six hyperbolic functions areactually one-to-one (the hyperbolic sine, tangent, cosecant, and cotangent). So, youcan apply Theorem 5.7 to conclude that these four functions have inverse functions.The other two (the hyperbolic cosine and secant) are one-to-one if their domains arerestricted to the positive real numbers, and for this restricted domain they alsohave inverse functions. Because the hyperbolic functions are defined in terms ofexponential functions, it is not surprising to find that the inverse hyperbolic functionscan be written in terms of logarithmic functions, as shown in Theorem 5.19.

Proof The proof of this theorem is a straightforward application of the properties ofthe exponential and logarithmic functions. For example, if

and

you can show that and which implies that is the inversefunction of f.

gg� f�x � x,f �g�x � x

g�x � ln�x � �x2 � 1

f�x � sinh x �ex � e�x

2

�sinh3 2x

6� C

�12�

�sinh 2x3

3 � � C

u � sinh 2x�cosh 2x sinh2 2x dx �12��sinh 2x2�2 cosh 2x dx

�cosh 2x sinh2 2x dx.

THEOREM 5.19 Inverse Hyperbolic Functions

���, 0 � �0, �csch�1 x � ln�1x

��1 � x2

x ��0, 1�sech�1 x � ln

1 � �1 � x2

x

���, �1 � �1, �coth�1 x �12

ln x � 1x � 1

��1, 1tanh�1 x �12

ln 1 � x1 � x

1, �cosh�1 x � ln�x � �x2 � 1 ���, �sinh�1 x � ln�x � �x2 � 1Domain Function

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SECTION 5.8 Hyperbolic Functions 393

The graphs of the inverse hyperbolic functions are shown in Figure 5.41.

The inverse hyperbolic secant can be used to define a curve called a tractrix orpursuit curve, as discussed in Example 5.

1

1

2

2

3

3

−1−2

−2

−3

−3

y = sinh−1 x

x

y

Domain: Range: ���, �

���, �

1

1

2

2

3

3

−1

−3

y = csch−1 x

x

y

Domain: Range: Figure 5.41

���, 0 � �0, ����, 0 � �0, �

−3 3

−2

y y1 2=

y y3 4=

2

Graphs of the hyperbolic tangent functionand the inverse hyperbolic tangent functionFigure 5.40

TECHNOLOGY You can use a graphing utility to confirm graphically theresults of Theorem 5.19. For instance, graph the following functions.

Hyperbolic tangent

Definition of hyperbolic tangent

Inverse hyperbolic tangent

Definition of inverse hyperbolic tangent

The resulting display is shown in Figure 5.40. As you watch the graphs beingtraced out, notice that and Also notice that the graph of is thereflection of the graph of in the line y � x.y3

y1y3 � y4.y1 � y2

y4 �12

ln 1 � x1 � x

y3 � tanh�1 x

y2 �ex � e�x

ex � e�x

y1 � tanh x

1

1

2

2

3

3

−1−1

−2

−2

−3

−3

y = cosh−1 x

x

y

Domain: Range: 0, �

1, �

1

1

2

2

3

3

−1−2 −1

−2

−3

−3

y = sech−1 x

x

y

Domain: Range: 0, �

�0, 1�

1

1

2

2

3

3

−1−2

−2

−3

−3

y = tanh−1 x

x

y

Domain: Range: ���, �

��1, 1

1

1

2

2

3

3

−1

−2

−3

y = coth−1 x

x

y

Domain: Range: ���, 0 � �0, �

���, �1 � �1, �

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394 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

EXAMPLE 5 A Tractrix

A person is holding a rope that is tied to a boat, as shown in Figure 5.42. As the personwalks along the dock, the boat travels along a tractrix, given by the equation

where is the length of the rope. If feet, find the distance the person mustwalk to bring the boat 5 feet from the dock.

Solution In Figure 5.42, notice that the distance the person has walked is given by

When this distance is

feet.

Differentiation and Integration of Inverse HyperbolicFunctions

The derivatives of the inverse hyperbolic functions, which resemble the derivatives ofthe inverse trigonometric functions, are listed in Theorem 5.20 with the correspondingintegration formulas (in logarithmic form). You can verify each of these formulas byapplying the logarithmic definitions of the inverse hyperbolic functions. (See Exercises99–101.)

� 41.27

� 20 ln�4 � �15 y1 � 20 sech�1

520

� 20 ln 1 � �1 � �1�42

1�4

x � 5,

� 20 sech�1 x

20.

y1 � y � �202 � x2 � �20 sech�1 x

20� �202 � x2� � �202 � x2

a � 20a

y � a sech�1 xa

� �a2 � x2

x

(0, y1)

(x, y)

10 20

x

2020

2 −

x2

y = 20 sech−1 − 202 − x2x20

Pers

on

y

A person must walk 41.27 feet to bring theboat 5 feet from the dock.Figure 5.42

THEOREM 5.20 Differentiation and Integration InvolvingInverse Hyperbolic Functions

Let be a differentiable function of

� du

u�a2 ± u2� �

1a

ln a � �a2 ± u2

u � C

� dua2 � u2 �

12a

ln a � ua � u � C

� C� du�u2 ± a2

� ln �u � �u2 ± a2

ddx

csch�1 u� ��u�

u �1 � u2

ddx

sech�1 u� ��u�

u�1 � u2

ddx

coth�1 u� �u�

1 � u2

ddx

tanh�1 u� �u�

1 � u2

ddx

cosh�1 u� �u�

�u2 � 1

ddx

sinh�1 u� �u�

�u2 � 1

x.u

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SECTION 5.8 Hyperbolic Functions 395

EXAMPLE 6 More About a Tractrix

For the tractrix given in Example 5, show that the boat is always pointing toward theperson.

Solution For a point on a tractrix, the slope of the graph gives the direction ofthe boat, as shown in Figure 5.42.

However, from Figure 5.42, you can see that the slope of the line segment connectingthe point with the point is also

So, the boat is always pointing toward the person. (It is because of this property thata tractrix is called a pursuit curve.)

EXAMPLE 7 Integration Using Inverse Hyperbolic Functions

Find

Solution Let and

EXAMPLE 8 Integration Using Inverse Hyperbolic Functions

Find

Solution Let and

� 1

4�5 ln �5 � 2x

�5 � 2x � C

12a

ln a � ua � u � C �

12� 1

2�5 ln �5 � 2x

�5 � 2x � � C

� dua2 � u2 � dx

5 � 4x2 �12� 2 dx

��5 2 � �2x2

u � 2x.a � �5

� dx5 � 4x2.

�1a

ln a � �a2 � u2

u � C � �12

ln 2 � �4 � 9x2

3x � C

� du

u�a2 � u2� dx

x�4 � 9x2� � 3 dx

�3x�4 � 9x2

u � 3x.a � 2

� dx

x�4 � 9x2.

m � ��202 � x2

x.

�x, y�0, y1

� ��202 � x2

x

��202

x�202 � x2�

x�202 � x2

� �20� 120�� 1

�x�20�1 � �x�202� � �12�� �2x

�202 � x2�y� �

ddx�20 sech�1

x20

� �202 � x2�

�x, y

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396 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

In Exercises 1–6, evaluate the function. If the value is not arational number, give the answer to three-decimal-placeaccuracy.

1. (a) 2. (a)

(b) (b)

3. (a) 4. (a)

(b) (b)

5. (a) 6. (a)

(b) (b)

In Exercises 7–12, verify the identity.

7.

8.

9.

10.

11.

12.

In Exercises 13 and 14, use the value of the given hyperbolicfunction to find the values of the other hyperbolic functions at

13. 14.

In Exercises 15–24, find the derivative of the function.

15. 16.

17. 18.

19. 20.

21. 22.

23. 24.

In Exercises 25–28, find an equation of the tangent line to thegraph of the function at the given point.

25. 26.

27. 28.

In Exercises 29–32, find any relative extrema of the function.Use a graphing utility to confirm your result.

29.

30.

31. 32.

In Exercises 33 and 34, show that the function satisfies the differential equation.

33.

34.

Linear and Quadratic Approximations In Exercises 35 and 36,use a computer algebra system to find the linear approximation

and the quadratic approximation

of the function at Use a graphing utility to graph thefunction and its linear and quadratic approximations.

35. 36.

Catenary In Exercises 37 and 38, a model for a power cablesuspended between two towers is given. (a) Graph the model,(b) find the heights of the cable at the towers and at themidpoint between the towers, and (c) find the slope of the modelat the point where the cable meets the right-hand tower.

37.

38.

In Exercises 39–50, find the integral.

39. 40.

41. 42. � sinh x1 � sinh2 x

dx�cosh2�x � 1 sinh�x � 1 dx

�cosh �x�x

dx�sinh�1 � 2x dx

y � 18 � 25 cosh x

25, �25 ≤ x ≤ 25

y � 10 � 15 cosh x

15, �15 ≤ x ≤ 15

a � 0f �x � cosh x,a � 0f �x � tanh x,

x � a.f

P2�x� � f �a� � f��a��x � a� � 12 f ��a��x � a�2

P1�x� � f �a� � f��a��x � a�

y� � y � 0y � a cosh x

y��� � y� � 0y � a sinh x

Differential EquationFunction

h�x � 2 tanh x � xg�x � x sech x

f �x � x sinh�x � 1 � cosh�x � 1�4 ≤ x ≤ 4f �x � sin x sinh x � cos x cosh x,

x

y

(0, 1)

−1−2 1 2

1

2

3

4

x

y

−1 1 2 3 4

1

4

5

(0, 1)

y � esinh xy � �cosh x � sinh x2

(1, 1)

1 2 3

1

2

3

x

y

−2−3 1 2 3−1

−2

−3

−4

2

x

y

(1, 0)

y � xcosh xy � sinh�1 � x2

g�x � sech2 3xf �t � arctan�sinh t

h�t � t � coth th�x �14

sinh 2x �x2

y � x cosh x � sinh xy � ln�tanh x2�

g�x � ln�cosh xf �x � ln�sinh xy � coth 3xy � sech�x � 1

tanh x �12

sinh x �32

x.

cosh x � cosh y � 2 cosh x � y

2 cosh

x � y2

sinh 3x � 3 sinh x � 4 sinh3 x

sinh 2x � 2 sinh x cosh x

sinh�x � y � sinh x cosh y � cosh x sinh y

cosh2 x �1 � cosh 2x

2

tanh2 x � sech2 x � 1

coth�1 3sech�1 23

csch�1 2cosh�1 2

tanh�1 0coth�ln 5sinh�1 0csch�ln 2sech 1tanh��2cosh 0sinh 3

E x e r c i s e s f o r S e c t i o n 5 . 8 See www.CalcChat.com for worked-out solutions to odd-numbered exercises.

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SECTION 5.8 Hyperbolic Functions 397

43. 44.

45. 46.

47. 48.

49. 50.

In Exercises 51–56, evaluate the integral.

51. 52.

53. 54.

55. 56.

In Exercises 57–64, find the derivative of the function.

57. 58.

59.

60.

61.

62.

63.

64.

Limits In Exercises 67–72, find the limit.

67. 68.

69. 70.

71. 72.

In Exercises 73–80, find the indefinite integral using theformulas of Theorem 5.20.

73. 74.

75. 76.

77. 78.

79. 80.

In Exercises 81–84, solve the differential equation.

81.

82.

83. 84.

Area In Exercises 85–88, find the area of the region.

85. 86.

87. 88.

In Exercises 89 and 90, evaluate the integral in terms of(a) natural logarithms and (b) inverse hyperbolic functions.

89. 90.

91. Chemical Reactions Chemicals A and B combine in a 3-to-1ratio to form a compound. The amount of compound beingproduced at any time is proportional to the unchanged amountsof A and B remaining in the solution. So, if 3 kilograms of A ismixed with 2 kilograms of B, you have

One kilogram of the compound is formed after 10 minutes. Findthe amount formed after 20 minutes by solving the equation

�3k16

dt � � dxx2 � 12x � 32

.

dxdt

� k�3 �3x4 ��2 �

x4� �

3k16

�x2 � 12x � 32.

tx

�1�2

�1�2

dx1 � x2��3

0

dx�x2 � 1

x

y

−2−4 2 4−2

2

6

8

4x

y

−1−2−3−4 1 2 3 4

−4

1234

y �6

�x2 � 4y �

5x�x 4 � 1

−2 −1−3 1 2 3

−2

−3

2

1

3

x

y

−1−2−3−4 1 2 3 4

0.20.40.6

1.21.4

x

y

y � tanh 2xy � sech x2

dydx

�1 � 2x4x � x2

dydx

�x3 � 21x

5 � 4x � x2

dydx

�1

�x � 1��4x2 � 8x � 1

dydx

�1

�80 � 8x � 16x2

� dx

�x � 1�2x2 � 4x � 8� 1

1 � 4x � 2x2 dx

� dx�x � 2�x2 � 4x � 8

� �14x � x2 dx

� �x�1 � x3

dx� 1�x�1 � x

dx

� x9 � x 4 dx� 1

�1 � e2x dx

limx→0�

coth xlimx→0

sinh x

x

limx→��

csch xlimx→�

sech x

limx→�

tanh xlimx→�

sinh x

y � x tanh�1 x � ln�1 � x2

y � 2x sinh�1�2x � �1 � 4x2

y � �csch�1 x2

y � tanh�1�sin 2x0 < x < ��4y � sech�1�cos 2x,

y � sinh�1�tan x

y � tanh�1 x2

y � cosh�1�3x

�ln 2

0 2e�x cosh x dx��2�4

0

2�1 � 4x2

dx

�4

0

1�25 � x2

dx�4

0

125 � x2 dx

�1

0 cosh2 x dx�ln 2

0 tanh x dx

� 2

x�1 � 4x2 dx� x

x 4 � 1 dx

� cosh x�9 � sinh2 x

dx�csch�1�x coth�1�xx2 dx

�sech3 x tanh x dx�x csch2 x2

2 dx

�sech2�2x � 1 dx�cosh xsinh x

dx

Writing About Concepts65. Discuss several ways in which the hyperbolic functions are

similar to the trigonometric functions.66. Sketch the graph of each hyperbolic function. Then identify

the domain and range of each function.

332460_0508.qxd 11/1/04 4:03 PM Page 397

398 CHAPTER 5 Logarithmic, Exponential, and Other Transcendental Functions

92. Vertical Motion An object is dropped from a height of 400 feet.

(a) Find the velocity of the object as a function of time (neglectair resistance on the object).

(b) Use the result in part (a) to find the position function.

(c) If the air resistance is proportional to the square of the veloc-ity, then where feet per secondper second is the acceleration due to gravity and is aconstant. Show that the velocity as a function of time is

by performing the following integration and simplifying theresult.

(d) Use the result in part (c) to find and give its interpretation.

(e) Integrate the velocity function in part (c) and find the position of the object as a function of Use a graphingutility to graph the position function when andthe position function in part (b) in the same viewing win-dow. Estimate the additional time required for the objectto reach ground level when air resistance is not neglected.

(f) Give a written description of what you believe wouldhappen if were increased. Then test your assertion witha particular value of

Tractrix In Exercises 93 and 94, use the equation of the tractrix

93. Find

94. Let be the tangent line to the tractrix at the point . If intersects the axis at the point show that the distancebetween and is

95. Prove that

96. Show that

97. Let and Show that

In Exercises 98–102, verify the differentiation formula.

98. 99.

100. 101.

102.ddx

sech x� � �sech x tanh x

ddx

sinh�1 x� �1

�x2 � 1

ddx

cosh�1 x� �1

�x2 � 1

ddx

sech�1 x� ��1

x�1 � x2

ddx

cosh x� � sinh x

�b

�b

ext dt �2 sinh bx

x.b > 0.x > 0

arctan�sinh x � arcsin�tanh x.

�1 < x < 1.tanh�1 x �12

ln�1 � x1 � x�,

a.QPQ,y-

LPL

dy�dx.

a > 0.y � a sech�1 xa

� �a2 � x2,

k.k

k � 0.01t.s

limt→�

v�t

� dv32 � kv2 � �� dt

v�t � ��32k

tanh��32k t

vk

�32dv�dt � �32 � kv2,

Section Project: St. Louis Arch

The Gateway Arch in St. Louis, Missouri was constructed using thehyperbolic cosine function. The equation used for construction was

where and are measured in feet. Cross sections of the arch areequilateral triangles, and traces the path of the centers of massof the cross-sectional triangles. For each value of the area of thecross-sectional triangle is (Source: Owner’s Manual for the Gateway Arch, Saint Louis, MO,by William Thayer)

(a) How high above the ground is the center of the highest triangle?(At ground level, )

(b) What is the height of the arch? (Hint: For an equilateral triangle,where is one-half the base of the triangle, and the

center of mass of the triangle is located at two-thirds the heightof the triangle.)

(c) How wide is the arch at ground level?

cA � �3c2,

y � 0.

A � 125.1406 cosh 0.0100333x.x,

�x, yyx

�299.2239 ≤ x ≤ 299.2239

y � 693.8597 � 68.7672 cosh 0.0100333x,

Putnam Exam Challenge

103. From the vertex of the catenary a lineis drawn perpendicular to the tangent to the catenary at a

point Prove that the length of intercepted by the axes isequal to the ordinate of the point

104. Prove or disprove that there is at least one straight line normalto the graph of at a point and alsonormal to the graph of at a point

[At a point on a graph, the normal line is the perpendicular tothe tangent at that point. Also, and

]

These problems were composed by the Committee on the Putnam Prize Competition.© The Mathematical Association of America. All rights reserved.

sinh x � �ex � e�x�2.cosh x � �ex � e�x�2

�c, sinh c.y � sinh x�a, cosh ay � cosh x

P.yLP.

Ly � c cosh �x�c�0, c

Roy

Oom

s/M

aste

rfile

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