6.7 Hyperbolic Functions
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Hyperbolic Functions
Certain even and odd combinations of the exponential
functions ex and e–x arise so frequently in mathematics and its applications that they deserve to be given special names.
In many ways they are analogous to the trigonometric functions, and they have the same relationship to the
hyperbola that the trigonometric functions have to the circle.
Hyperbolic Functions
For this reason they are collectively called hyperbolic functions and individually called hyperbolic sine,
hyperbolic cosine, and so on.
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Hyperbolic Functions
The hyperbolic functions satisfy a number of identities that are similar to well-known trigonometric identities.
We list some of them here and leave most of the proofs to the exercises.
Example 1
Prove (a) cosh2x – sinh2x = 1 and (b) 1 – tanh2x = sech2x.
Solution:
(a) cosh2x – sinh2x =
=
=
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Example 1 – Solution
(b) We start with the identity proved in part (a):
cosh2x – sinh2x = 1
If we divide both sides by cosh2x, we get
or
cont’d
Hyperbolic Functions
The derivatives of the hyperbolic functions are easily computed. For example,
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Hyperbolic Functions
We list the differentiation formulas for the hyperbolic functions as Table 1.
Example 2
Any of these differentiation rules can be combined with the Chain Rule. For instance,
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Inverse Hyperbolic Functions
Inverse Hyperbolic Functions
The sinh and tanh are one-to-one functions and so they have inverse functions denoted by sinh–1 and tanh–1. The cosh is not one-to-one, but when restricted to the domain [0, ) it becomes one-to-one.
The inverse hyperbolic cosine function is defined as the inverse of this restricted function.
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Inverse Hyperbolic Functions
We can sketch the graphs of sinh–1, cosh–1, and tanh–1 in Figures 8, 9, and 10.
Figure 8
domain = range =
Figure 9
domain = [1, ) range = [0, )
Inverse Hyperbolic Functions
domain = (–1, 1) range =
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Inverse Hyperbolic Functions
Since the hyperbolic functions are defined in terms of exponential functions, it’s not surprising to learn that the inverse hyperbolic functions can be expressed in terms of logarithms.
In particular, we have:
Example 3
Show that sinh–1x = Solution:
Let y = sinh–1x. Then
so ey – 2x – e–y = 0 or, multiplying by ey,
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Example 3 – Solution
This is really a quadratic equation in ey: (ey)2 – 2x(ey) – 1 = 0 Solving by the quadratic formula, we get
Note that ey > 0, but
cont’d
Example 3 – Solution
Thus the minus sign is inadmissible and we have
Therefore
This shows that
cont’d
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Inverse Hyperbolic Functions
The inverse hyperbolic functions are all differentiable because the hyperbolic functions are differentiable.
Example 4
Prove that Solution:
Let y = sinh–1x. Then sinh y = x. If we differentiate this equation implicitly with respect to x, we get
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Example 4 – Solution
Since cosh2y – sinh2y = 1 and cosh y ≥ 0, we have cosh y = so
cont’d
