Hyperbolic Identities & Equations (Edexcel A Level Further Maths: Core Pure)

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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
    • cos x equals 1 half open parentheses straight e to the power of straight i x end exponent plus straight e to the power of negative straight i x end exponent close parentheses
    • sin x equals fraction numerator 1 over denominator 2 straight i end fraction open parentheses straight e to the power of straight i x end exponent minus straight e to the power of negative straight i x end exponent close parentheses
  • Compare these with the definitions for the hyperbolic functions
    • cosh x equals 1 half open parentheses straight e to the power of x plus straight e to the power of negative x end exponent close parentheses
    • sinh x equals 1 half open parentheses straight e to the power of x minus straight e to the power of negative x end exponent close parentheses
  • Therefore they can be related using the identities
    • cos x identical to cosh left parenthesis straight i x right parenthesis or cosh x identical to cos open parentheses straight i x close parentheses
    • sin x identical to negative isinh left parenthesis straight i x right parenthesis or sinh x identical to negative isin open parentheses straight i x close parentheses

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. cos squared x plus sin squared x identical to 1 but  cosh squared x minus sinh squared x identical to 1
    • 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
    • cosh squared x minus sinh squared x identical to 1
    • sinh 2 x identical to 2 sinh x cosh x
    • cosh 2 x identical to cosh squared x plus sinh squared x
      • These are listed in the formulae booklet
  • Other identities include
    • cosh 2 x identical to 2 cosh squared x minus 1 identical to 1 plus 2 sinh squared x
    • sinh open parentheses A plus-or-minus B close parentheses equals sinh A cosh B plus-or-minus sinh B cosh A
    • cosh open parentheses A plus-or-minus B close parentheses identical to cosh A cosh B plus-or-minus sinh A sinh B
  • The harmonic identities can also be used with hyperbolic functions
    • a cosh x plus-or-minus b sinh x equals R cosh open parentheses x plus-or-minus alpha close parentheses
    • a sinh x plus-or-minus b cosh x equals R sinh open parentheses x plus-or-minus alpha close parentheses
  • 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
  • cosech x equals fraction numerator 1 over denominator sinh x end fraction equals fraction numerator 2 over denominator straight e to the power of x minus straight e to the power of negative x end exponent end fraction (Pronounced “coshec”)
  • sech x equals fraction numerator 1 over denominator cosh x end fraction equals fraction numerator 2 over denominator straight e to the power of x plus straight e to the power of negative x end exponent end fraction (Pronounced “shec”)
  • coth x equals fraction numerator 1 over denominator tanh x end fraction equals fraction numerator cosh x over denominator sinh x end fraction equals fraction numerator straight e to the power of 2 x end exponent plus 1 over denominator straight e to the power of 2 x end exponent minus 1 end fraction(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
  • This approach can lead to what seems like a dead-end
    • 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
    • Use your calculator if exact answers are not required
  • 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 cosh x equals k
      • x equals plus-or-minus arcosh k
      • This can be written in logarithmic form as plus-or-minus ln open parentheses k plus square root of k squared minus 1 end root close parentheses
      • This can be shown to be equivalent to ln open parentheses k plus-or-minus square root of k squared minus 1 end root close parentheses  
    • If k=1 then the only solution to cosh x equals k is x=0
    • If k<1 then there are no real solutions to cosh x equals k

Examiner 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 sinh x and cosh x prove the identity cosh 2 x equals 1 plus 2 sinh squared x.

al-fm-4-1-3-we-solution-a

b)
Find the real solutions, as exact values, to the equation cosh 2 x equals 15 plus 3 sinh x

al-fm-4-1-3-we-solution-b

 

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Paul

Author: Paul

Expertise: Maths

Paul has taught mathematics for 20 years and has been an examiner for Edexcel for over a decade. GCSE, A level, pure, mechanics, statistics, discrete – if it’s in a Maths exam, Paul will know about it. Paul is a passionate fan of clear and colourful notes with fascinating diagrams – one of the many reasons he is excited to be a member of the SME team.