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Hyperbolic function question

  1. Sep 20, 2009 #1
    Hey guys,

    I was doing some work on hyperbolic functions and teaching myself to solve some equations. One of the questions in the book really has me stumped:

    Express using exponential definitions of cosh(x) and sinh(x) find the exact solution of:
    tanh(x) + sinh(x) = 3

    I had a go at solving it and this is how far I got:

    2tanh(x) + 2sinh(x) = 6

    2(e2x -1)/(e2x -1) + ex - e-x = 6

    e3x - 4e2x - 8 - e-x = 0

    e4x - 4e3x - 8ex - 1 = 0


    then if y=ex

    y4 - 4y3 - 8y -1 = 0


    After this I get stuck. I can't find any factors in order to solve it using factor theorem so I'm guessing I'm going to get some wierd solutions - but the question asks specifically for exact answers?

    Would anyone mind please helping me out? (I really hope I haven't made some pathetic little mistake but I really can't see anything...)


    Thanks in advance,
    Oscar
     
  2. jcsd
  3. Sep 20, 2009 #2

    uart

    User Avatar
    Science Advisor

    Your working looks correct and a numerical solution would be easy enough but yeah I cant see any way to get an "exact" solution here either.

    Here's another approach which you may find useful (btw it leads to a slightly simpler equation but still no "exact" solution that I can see - though maybe someone else will).

    Just about every trig identity for standard trig functions has a counterpart for the hyperbolic trig's. In this case it's the hyperbolic counterpart of the t=tan(x/2) identities that are useful. These identies are :

    [tex]t = \tanh(x/2)[/tex]

    [tex]\tanh(x) = 2t/(1+t^2)[/tex]

    [tex]\sinh(x) = 2t/(1-t^2)[/tex]

    [tex]\cosh(t) = (1+t^2)/(1-t^2)[/tex]

    Using the identities for tanh and sinh above in your equation it's pretty easy to obtain :

    [tex]3t^4 + 4t - 3 = 0[/tex]

    Numerically t = 0.63106 and [itex]x = 2 \tanh^{-1}(t)[/itex] = 1.4863 to 5 sig figures. This solution also works in your equation (and of course in the original hyperbolic equation) so we can safely assume it is "correct" - though unfortunately just a numerical solution.
     
    Last edited: Sep 20, 2009
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