# Hyperbolic Functions

The hyperbolic *cosine function* is defined to be:

cosh(x) = (e^{x}+ e^{-x})/2

And the hyperbolic *sine function* is defined as:

sinh(x) = (e^{x}- e^{-x})/2

From the above definition we can see the following properties of the cosh(x) and sinh(x) functions:

cosh(x) is an even function:

cosh(-x) = (e^{-x}+ e^{x})/2 = (e^{x}+ e^{-x})/2 = cosh(x)

Hence the function is even function.

Similarly the sinh(x) is an odd function:

sinh(-x) = (e^{-x}- e^{x})/2 = -((e^{x}- e^{-x})/2) = -sinh(x)

Hence sinh(x) function is an odd function.

Range of cosh(x):

cosh(x) = (e^{-x}+ e^{x})/2 Let y = e^{x}So, cosh(x) = (y + 1/y)/2 . Now (y + 1/y) ≥ 2 for y>0. Hence cosh(x) ≥ 1

Therefore, range of cosh(x) is [1, ∞).

Range of sinh(x):

sinh(x) = (e^{x}- e^{-x})/2 Let sinh(x) = y. So y = (e^{2x}- 1)/2e^{x}2ye^{x}= e^{2x}- 1 e^{2x}- 2ye^{x}+ 1 = 0 x = ln(y + √(y^{2}+ 1))

Clearly since x is in (-∞, ∞), y has to lie in (-∞, ∞) so that the above log gives all real numbers.

Analogy between *hyperbolic sines* and *cosines* and normal *trigonometric sines* and *cosines*.

**Osbourne’s Rule:**

This rule states that the normal trigonometric identities in sines and cosines remain the same even with hyperbolic sines and cosines, i.e., cos(x) can be replaced by cosh(x) and sin(x) can be replaced by sinh(x).But it is to be remembered that when there is a multiplication of two sine functions there is a sign change in the identity.

For example,

sin(2A) = 2sin(A)cos(A)

It remains the same in hyperbolic functions, i.e.,

sinh(2x) = 2sinh(x)cosh(x) But, cos(2A) = 1 - 2sin^{2}(x)

which becomes,

cosh(2x) = 1 + 2sinh^{2}(x)

There is also analogy in geometry. While (cos(t), sin(t)) represent the points on a unit circle, (cosh(x), sinh(x)) represent points on a hyperbola with x-intercept = 1.

Identity:

cosh^{2}(x) - sinh^{2}(x) = 1

Proof: = L.H.S = ((e^{x}+ e^{-x})/2)^{2}- ((e^{x}- e^{-x})/2)^{2}= ((e^{2x}+ e^{-2x}+ 2)/4) - ((e^{2x}+ e^{-2x}- 2)/4) = 4/4 = 1

Other Hyperbolic Functions:

tanh(x) = sinh(x)/cosh(x) coth(x) = cosh(x)/sinh(x) cosech(x) = 1/sinh(x) sech(x) = 1/cosh(x)

Derivatives of Hyperbolic Functions:

= d(cosh(x))/dx = d((e^{x}+ e^{-x})/2)/dx = 1/2(d(e^{x}+ e^{-x})/dx) = 1/2(e^{x}- e^{-x}) = sinh(x)

Similarly,

d(sinh(x))/dx = cosh(x)

You can see the difference between derivatives of sin(x) and cos(x) and sinh(x) and cosh(x). So you can carry out any operation for hyperbolic functions by just replacing them with their defined functions.

## Recommended Posts:

- Mathematics | Classes (Injective, surjective, Bijective) of Functions
- Mathematics | Representation of Boolean Functions
- Mathematics | Total number of possible functions
- Discrete Maths | Generating Functions-Introduction and Prerequisites
- Mathematics | Unimodal functions and Bimodal functions
- Activation Functions
- Inverse functions and composition of functions
- Mathematics | Generating Functions - Set 2
- Digital Logic | Self dual functions
- Digital Logic | Number of Boolean functions
- Digital Logic | Number of possible Functions
- Second Order Linear Differential Equations
- Lagrange Multipliers
- Gauss's Forward Interpolation

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.