Hyperbolic function question

  • Thread starter 2^Oscar
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  • #1
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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
 

Answers and Replies

  • #2
uart
Science Advisor
2,776
9
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.
 
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