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Hyperbolic functions (CheatSheet) Final exam
1 Intro
For historical reasons hyperbolic functions have little or no room at all in the syllabus of a calculus
course, but as a matter of fact they have the same dignity as trigonometric functions. Unfortu-
nately this can be completely understood only if you have some knowledge of the complex
numbers. Roughly speaking ordinary trigonometric functions are trigonometric functions of purely
real num- bers, and hyperbolic functions are trigonometric functions of purely imaginary numbers.
For the moment we have to postpone this discussion to the end of Calc3 or Calc4, but still we
should be aware of the fact that the impressive similarity between trig formulas and hyperbolic
formulas is not a pure coincidence.
Most of the formulas that follow correspond precisely to a trig formula or they differ by at most
a change of sign. For each formula I will explicitly state if some change of sign occurs or not (the
different sign is marked in green).
The main purpose of this paper is not to give you a bunch of formulas to memorize, but to
make you aware of the fact that hyperbolic formulas are just like trig formulas up to signs; and
correct signs can always be checked with some very quick calculation.
2 Definitions
Definition of hyperbolic sine and cosine:
sinh x =
cosh x
ex^ โ
e โ x 2
ex^ + e โ x 2
There are two equivalent formulas for sine and cosine (Eulerโs formulas) but they require some
knowledge of the complex numbers:
sin x =
cos x =
eix^ โ e โ ix 2 i
eix^ + e โ ix 2
where i =
โ 1 or if you prefer i^2 = โ1. Substituting x with ix in these two formulas and keeping
in mind that i^2 = โ 1 itโs immediate to deduce that cosh x = cos ( ix ) and sinh x = โ i sin
( ix ) (I mention this just for the sake of completeness and because itโs fun!). From the definition
of hyperbolic sine and cosine we define hyperbolic tangent, cotangent, secant, cosecant in the
same
way we did for trig functions:
tanh x =
coth x =
sech x =
csch x =
sinh x cosh x cosh x sinh x 1 cosh x 1 sinh x
3 Basic properties
First of all we notice that hyperbolic functions have the same parity as the corresponding trig
functions:
sinh (โ x ) = โ sinh x (1)
cosh (โ x ) = cosh x (2)
tanh (โ x ) = โ tanh x (3) coth (โ x ) = โ coth x (4)
sech (โ x ) = sech x (5) csch (โ x ) = โ csch x (6)
(7)
All these formulas follow immediately from the definitions we gave in the previous section. Unlike
trig functions, hyperbolic functions are not periodic!
Using the definition of hyperbolic sine and cosine itโs possible to derive identities similar to
cos^2 x + sin^2 x = 1 and tan^2 x + 1 = sec^2 x :
cosh^2 x โ sinh^2 x = 1 (8)
tanh^2 x + sech^2 x = +1 (9)
These identities do not require Pythagorasโ theorem, they can be derived from the definition with
a direct calculation and using properties of the exponential.
4 Addition formulas
Unlike the case of ordinary addition trig formulas, the two basic addition formulas for hyperbolic
functions can be retrieved immediately from the definition.
sinh ( x + y ) = sinh x cosh y + cosh x sinh y (10) cosh ( x + y ) = cosh x cosh y + sinh x sinh y (11)
Combining these formulas with ( 1 ) we easily derive the following:
sinh ( x โ y ) = sinh x cosh y โ cosh x sinh y (12)
cosh ( x โ y ) = cosh x cosh y โ sinh x sinh y (13)
(^1) You donโt need to choose the sign in front of the radical since cosh is always positive
2 1 โ x
2 x
The inverse of an hyperbolic function can always be written as the logarithm of an algebraic^2
function:
arsinh x = ln ( x + x^2 + 1) , Domain=( , + ), Range=( , + ) (23)
arcosh x = ln ( x +
x^2 โ 1) , Domain=[1 , +โ), Range=[0 , +โ) (24)
artanh x =
ln
1 + x , Domain=(โ 1 , 1), Range=(โโ , +โ) (25)
Remember that the domain of the inverse is the range of the original function, and the range of the
inverse is the domain of the original function. To retrieve these formulas we rewrite the definition
of the hyperbolic function as a degree two polynomial in ex ; then we solve for ex^ and invert the
exponential. For example:
y = sinh x
ex^ โ
e โ x 2
โ e โ 2 ye โ 1 = 0 โ e = y ยฑ
y^2 + 1
and since the exponential must be positive we select the positive sign.
7 Derivatives
The calculation of the derivative of an hyperbolic function is completely straightforward, so I will
just report a list of formulas with no additional comments:
d sinh x = cosh x (26) dx d cosh x = + sinh x (27) dx d tanh x = sech^2 x (28) dx d coth x = csch^2 x (29) dx d
dx
sech x = โ tanh x sech x (30) d
dx
csch x = โ coth x csch x (31)
Formulas for the derivative of an inverse hyperbolic function can be quickly calculated from ( 23 )
using basic properties of derivatives. They can also be calculated using the formula for the
derivative of the inverse:
d 1
dx
arsinh x = โ
d + 1
dx
arcosh x = โ x^2 โ 1
d 1
dx
artanh x = 1 โ x^2
(^2) An algebraic function is a function containing the four operations and radicals only.
x x
