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 January 1st, 2017, 06:46 AM #1 Newbie   Joined: Oct 2015 From: huddersfield Posts: 16 Thanks: 1 hyperbolic identities Prove the hyperbolic identities coth x ≡ cosech 2x + tanh x
January 1st, 2017, 08:43 AM   #2
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Quote:
 Originally Posted by greeny93 Prove the hyperbolic identities coth x ≡ cosech 2x + tanh x
check your identity equation. should be ...

$\coth{x} = 2\text{csch}(2x) + \tanh{x}$

 January 1st, 2017, 06:45 PM #3 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,437 Thanks: 871 Math Focus: Elementary mathematics and beyond Use the identity $\sinh2x=2\sinh x\cosh x$
January 4th, 2017, 10:55 AM   #4
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 Originally Posted by skeeter check your identity equation. should be ... $\coth{x} = 2\text{csch}(2x) + \tanh{x}$
where do i go from there?

 January 4th, 2017, 11:34 AM #5 Math Team   Joined: Jul 2011 From: Texas Posts: 2,426 Thanks: 1195 $\coth{x} = 2\text{csch}(2x) + \tanh{x}$ $\coth{x} = \dfrac{2}{\sinh(2x)} + \tanh{x}$ use greg1313's recommendation ... $\coth{x} = \dfrac{2}{2\sinh{x}\cosh{x}} + \tanh{x}$ $\coth{x} = \dfrac{1}{\sinh{x}\cosh{x}} + \dfrac{\sinh{x}}{\cosh{x}}$ $\coth{x} = \dfrac{1}{\sinh{x}\cosh{x}} + \dfrac{\sinh^2{x}}{\sinh{x}\cosh{x}}$ $\coth{x} = \dfrac{1+ \sinh^2{x}}{\sinh{x}\cosh{x}}$ recall $1 = \cosh^2{x}-\sinh^2{x}$ ... can you finish from here?
 January 4th, 2017, 11:46 AM #6 Newbie   Joined: Oct 2015 From: huddersfield Posts: 16 Thanks: 1 skeeter, you seem to have a very good understanding of this topic, whereas I believe mine is not so good; where is the best place to gain a better understanding of this subject? I can't seem to grasp the next stages after your answers. Last edited by skipjack; January 4th, 2017 at 12:08 PM.
 January 4th, 2017, 12:45 PM #7 Math Team   Joined: Jul 2011 From: Texas Posts: 2,426 Thanks: 1195 Experience has been my best teacher. You know what I mean by that statement. Some good tips (in the context of regular trig identities, but they still apply to the hyperbolic ones) ... Tips for Trig Identities Basic hyperbolic trig identities you learn, or at the very least become somewhat familiar. These are the ones that get you started ... The Hyperbolic Identities
 January 4th, 2017, 05:46 PM #8 Global Moderator   Joined: Dec 2006 Posts: 16,775 Thanks: 1236 The suggested methods assume you already know that sinh 2x ≡ 2sinh x cosh x and 1 ≡ cosh²x - sinh²x. Have you been taught those identities? If you have, and if x is not zero, coth x = cosh x / sinh x = 2cosh²x / (2sinh x cosh x) $\ \ \ \ \ \ \ \ \ \$ = (2 + 2sinh²x) / (2sinh x cosh x) $\ \ \ \ \ \ \ \ \ \$ = 2/sinh 2x + sinh x / cosh x, etc. If you prefer not to assume the identities mentioned above, you can instead use the equations sinh x = (e^x - e^(-x))/2 and cosh x = (e^x + e^(-x))/2, along with coth x = cosh x / sinh x and tanh x = sinh x / cosh x, etc.

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