# Edexcel A Level Further Maths: Core Pure:复习笔记4.1.3 Hyperbolic Identities & Equations

### Hyperbolic Identities & Equations

#### Are there identities linking the hyperbolic functions to the circular trig functions?

• Yes - these can be seen using de Moivre's Theorem to write
• Compare these with the definitions for the hyperbolic functions
• Therefore they can be related using the identities

#### What are the hyperbolic identities?

• In general, the hyperbolic identities are the same as the circular trigonometric identities except where there is a product of an even number of sinh terms, in which case the term changes sign
• e.g.
• This is referred to as Osborn’s Rule
• This occurs because of the connection with the imaginary number i
• All the circular trigonometric identities can be used with hyperbolic functions
• The main hyperbolic identities you are likely to need are
• These are listed in the formulae booklet
• Other identities include
• The harmonic identities can also be used with hyperbolic functions
• Hyperbolic identities involving tanhx exist
• They are not normally used as it is easier to use sinhx, coshx and their definitions
• If you do use tanhx identities, be careful with implied or ‘hidden’ products of sinhx (e.g.  tanh2x)
• You can prove these identities by using the definitions of the hyperbolic functions in terms of e

#### Do reciprocal hyperbolic functions and identities exist?

• Yes! However, It is usually easier to deal with identities and equations involving these in terms of sinhx, coshx and their definitions
• (Pronounced “coshec”)
• (Pronounced “shec”)
• (Pronounced “cough”)

#### How do I use hyperbolic identities to prove other identities?

• Start with the LHS and use the hyperbolic identities to rearrange into the RHS
• In such cases simplify the LHS as far as possible, ideally so that it is in terms of sinhx and/or coshx only
• Then use the sinhx and coshx definitions to write the LHS in terms of e
• Repeat this for the RHS so that the LHS and RHS ‘meet in the middle’

#### How do I solve equations involving hyperbolic functions?

• Use identities to create an equation in terms of sinhx or coshx only
• This should be a familiar equation to solve – linear, quadratic, etc
• Find exact answers in terms of natural logarithms
• Using the inverse hyperbolic functions definitions
• As with circular trigonometric equations, do not cancel hyperbolic terms, rearrange so the equation equals zero and factorise
• When solving equations be careful when solving coshx = k (for constant k)
• cosh-1x is not necessarily the same as arcoshx
• arcoshx is, strictly speaking, referring to the inverse function of coshx such that coshx is a one-to-one function
• Using the graph you can see that the for k>1 there are two solutions to
• This can be written in logarithmic form as
• This can be shown to be equivalent to
• If k=1 then the only solution to is x=0
• If k<1 then there are no real solutions to

#### Exam Tip

• You can use the A Level Maths section of the formula booklet to remind you of trigonometric identities (such as sin(A±B)) which you can then adapt for the hyperbolic trig functions – don’t limit yourself to just the Further Maths section

#### Worked Example

a) Using the definitions of  prove the identity .

b) Find the real solutions, as exact values, to the equation .