# 3 DIFFERENTIATION RULES DIFFERENTIATION RULES Certain even and

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3 DIFFERENTIATION RULES

DIFFERENTIATION RULES 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.

DIFFERENTIATION RULES 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. § For this reason, they are collectively called hyperbolic functions and individually called hyperbolic sine, hyperbolic cosine, and so on.

DIFFERENTIATION RULES 3. 11 Hyperbolic Functions In this section, we will learn about: Hyperbolic functions and their derivatives.

DEFINITION

HYPERBOLIC FUNCTIONS The graphs of hyperbolic sine and cosine can be sketched using graphical addition, as in these figures.

HYPERBOLIC FUNCTIONS Note that sinh has domain and range , whereas cosh has domain and range.

HYPERBOLIC FUNCTIONS The graph of tanh is shown. § It has the horizontal asymptotes y = ± 1.

APPLICATIONS Some mathematical uses of hyperbolic functions will be seen in Chapter 7. Applications to science and engineering occur whenever an entity such as light, velocity, or electricity is gradually absorbed or extinguished. § The decay can be represented by hyperbolic functions.

APPLICATIONS The most famous application is the use of hyperbolic cosine to describe the shape of a hanging wire.

APPLICATIONS It can be proved that, if a heavy flexible cable is suspended between two points at the same height, it takes the shape of a curve with equation y = c + a cosh(x/a) called a catenary. § The Latin word catena means ‘chain. ’

APPLICATIONS Another application occurs in the description of ocean waves. § The velocity of a water wave with length L moving across a body of water with depth d is modeled by the function where g is the acceleration due to gravity.

HYPERBOLIC IDENTITIES The hyperbolic functions satisfy a number of identities that are similar to well-known trigonometric identities.

HYPERBOLIC IDENTITIES We list some identities here.

HYPERBOLIC FUNCTIONS Prove: a. cosh 2 x – sinh 2 x = 1 b. 1 – tanh 2 x = sech 2 x Example 1

HYPERBOLIC FUNCTIONS Example 1 a

HYPERBOLIC FUNCTIONS Example 1 b We start with the identity proved in (a): cosh 2 x – sinh 2 x = 1 § If we divide both sides by cosh 2 x, we get:

HYPERBOLIC FUNCTIONS The identity proved in Example 1 a gives a clue to the reason for the name ‘hyperbolic’ functions, as follows.

HYPERBOLIC FUNCTIONS If t is any real number, then the point P(cos t, sin t) lies on the unit circle x 2 + y 2 = 1 because cos 2 t + sin 2 t = 1. § In fact, t can be interpreted as the radian measure of in the figure.

HYPERBOLIC FUNCTIONS For this reason, the trigonometric functions are sometimes called circular functions.

HYPERBOLIC FUNCTIONS Likewise, if t is any real number, then the point P(cosh t, sinh t) lies on the right branch of the hyperbola x 2 - y 2 = 1 because cosh 2 t - sin 2 t = 1 and cosh t ≥ 1. § This time, t does not represent the measure of an angle.

HYPERBOLIC FUNCTIONS However, it turns out that t represents twice the area of the shaded hyperbolic sector in the first figure. § This is just as in the trigonometric case t represents twice the area of the shaded circular sector in the second figure.

DERIVATIVES OF HYPERBOLIC FUNCTIONS The derivatives of the hyperbolic functions are easily computed. § For example,

DERIVATIVES Table 1 We list the differentiation formulas for the hyperbolic functions here.

DERIVATIVES Note the analogy with the differentiation formulas for trigonometric functions. § However, beware that the signs are different in some cases.

DERIVATIVES Example 2 Any of these differentiation rules can be combined with the Chain Rule. § For instance,

INVERSE HYPERBOLIC FUNCTIONS You can see from the figures that sinh and tanh are one-to-one functions. § So, they have inverse functions denoted by sinh-1 and tanh-1.

INVERSE FUNCTIONS This figure shows that cosh is not one-to-one. However, when restricted to the domain [0, ∞], it becomes one-to-one.

INVERSE FUNCTIONS The inverse hyperbolic cosine function is defined as the inverse of this restricted function.

INVERSE FUNCTIONS Definition 2 § The remaining inverse hyperbolic functions are defined similarly.

INVERSE FUNCTIONS By using these figures, we can sketch the graphs of sinh-1, cosh-1, and tanh-1.

INVERSE FUNCTIONS The graphs of sinh-1, cosh-1, and tanh-1 are displayed.

INVERSE 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.

INVERSE FUNCTIONS In particular, we have: Defns. 3, 4, and 5

INVERSE FUNCTIONS Example 3 Show that § Let y = sinh-1 x. Then, § So, ey – 2 x – e-y = 0 § Or, multiplying by ey, e 2 y – 2 xey – 1 = 0 § This is really a quadratic equation in ey: (ey)2 – 2 x(ey) – 1 = 0 .

INVERSE FUNCTIONS Example 3 Solving by the quadratic formula, we get: § Note that ey > 0, but (because ). § So, the minus sign is inadmissible and we have: § Thus,

DERIVATIVES Table 6

DERIVATIVES The inverse hyperbolic functions are all differentiable because the hyperbolic functions are differentiable. § The formulas in Table 6 can be proved either by the method for inverse functions or by differentiating Formulas 3, 4, and 5.

DERIVATIVES E. g. 4—Solution 1 Prove that . § Let y = sinh-1 x. Then, sinh y = x. § If we differentiate this equation implicitly with respect to x, we get: § As cosh 2 y - sin 2 y = 1 and cosh y ≥ 0, we have: § So,

DERIVATIVES From Equation 3, we have: E. g. 4—Solution 2

DERIVATIVES Find Example 5 . § Using Table 6 and the Chain Rule, we have: