Hyperbolic trigonometric functions in terms of e ?

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Discussion Overview

The discussion centers on the definition and properties of hyperbolic trigonometric functions, specifically their representation in terms of the natural exponential function, e^x. Participants explore the relationship between hyperbolic functions and their trigonometric counterparts, as well as their mathematical significance and applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the definitions of hyperbolic functions, such as cosh(x) and sinh(x), are derived from the exponential function, specifically as cosh x = (e^x + e^-x)/2 and sinh x = (e^x - e^-x)/2.
  • Others suggest that the relationship between hyperbolic functions and standard trigonometric functions can be understood through analogous formulas, noting that removing the imaginary unit from the definitions of cosine and sine yields the hyperbolic functions.
  • A participant mentions that cosh(a), where a represents the area of a hyperbolic sector of a unit hyperbola, corresponds to the x-coordinate, indicating a geometric interpretation of the hyperbolic functions.
  • Another contribution discusses the fundamental solutions to the differential equations y'' + y = 0 and y'' - y = 0, linking hyperbolic functions to these solutions and their properties.
  • It is noted that hyperbolic functions satisfy the identity cosh²(x) - sinh²(x) = 1, paralleling the circular identity cos²(x) + sin²(x) = 1 for trigonometric functions.

Areas of Agreement / Disagreement

Participants express various viewpoints on the definitions and implications of hyperbolic functions, with no clear consensus reached. The discussion includes both supportive and contrasting perspectives on the relationships between hyperbolic and trigonometric functions.

Contextual Notes

Some definitions and interpretations of hyperbolic functions depend on specific mathematical contexts, such as geometric representations or differential equations, which may not be universally agreed upon.

Point Conception
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cosh x= e^x+e^-x/2
sinh x= e^x-e^-x/2

Can someone explain why the hyperbolic trigonometric functions are defined in terms of the natural exponential function, e^x ?
 
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Well, tell us the definition of cosh you want to use. Then we can work on why it is equal to (exp(x)+exp(-x))/2 ...
 
It probably has to do with analogous formulas for the standard trig functions:

cos x = (eix + e-ix)/2
sin x = (eix - e-ix)/2​

Simply remove the i's and they become the hyperbolic functions.
 
g_edgar said:
Well, tell us the definition of cosh you want to use. Then we can work on why it is equal to (exp(x)+exp(-x))/2 ...

That cosh (a) ,( a = area of hyperbolic sector of unit hyperbola )
is equal to the x coordinate.
see my thread starter: (https://www.physicsforums.com/showthread.php?t=336897 )
also linked below.
 
Last edited by a moderator:
morrobay said:
That cosh (a) ,( a = area of hyperbolic sector of unit hyperbola )
is equal to the x coordinate.
see my thread starter: (https://www.physicsforums.com/showthread.php?t=336897 )
also linked below.

g_edgar said:
Well, tell us the definition of cosh you want to use. Then we can work on why it is equal to (exp(x)+exp(-x))/2 ...

Whats up Doc ?
 
Last edited by a moderator:
It is also worth noting that the "fundamental solutions at x= 0" to the differential equation y"+ y= 0, the solutions such that y1(0)= 1, y1'(0)= 0 and y2(0)= 0, y2'(0)= 0, are y1(x)= cos(x) and y2(x)= sin(x).

They are so called because is y is any function of x satisfying that differential equation, then y(x)= y(0)cos(x)+ y'(0) sin(x). That is, the coefficients in the linear combination are just y(0) and y'(0).

The fundamental solutions for y"- y= 0, at x= 0, are, similarly, cosh(x) and sinh(x).

Of course, ex and e-x are also independent solutions of that equation so they can be written in terms of each other.
 
Well, they parametrize a very basic hyperbola:

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

while the trigonometric cosines and sines describe a very basic circle:

cos^{2}(x)+sin^{2}(x)=1
 

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