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Derivative help

  1. Feb 13, 2009 #1
    derivative help!!!!!!!

    1. The problem statement, all variables and given/known data
    find f'(x) sinhx/coshx


    2. Relevant equations
    this is what i got but im unsure if its wrong could anyone help me pleasee


    3. The attempt at a solution
    coshx(sinhx)-(coshx)coshx/coshx......... is this right if not help please
     
  2. jcsd
  3. Feb 13, 2009 #2

    Dick

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    Science Advisor
    Homework Helper

    Re: derivative help!!!!!!!

    How are you getting that? Write down the quotient rule for the derivative of f(x)/g(x) and check the parts.
     
  4. Feb 13, 2009 #3
    Re: derivative help!!!!!!!

    ok i took a look at the quotient rule and was completely wrong how does this look.....

    coshx(coshx)-sinhx(sinhx)/(coshx)^2.............. (Coshx)^2-(Sinhx)^2/(Coshx)^2..........the (coshx)^2 cancel out and you are left with -(sinhx)^2????????
     
  5. Feb 13, 2009 #4

    jgens

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    Gold Member

    Re: derivative help!!!!!!!

    Still not right. Cosh(x)^2 does not simply cancel out and vanish. If I asked you what 5/5 was equivalent to would you say zero?

    To get the correct answer use the identity cosh(x)^2 - sinh(x)^2 = 1 or the identity 1 - tanh(x)^2 = sech(x)^2
     
  6. Feb 13, 2009 #5
    Re: derivative help!!!!!!!

    ooooooo the identity oo ok so its 1/(coshx)^2?????????????????
     
  7. Feb 13, 2009 #6

    jgens

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    Gold Member

    Re: derivative help!!!!!!!

    Can you simplify that further?
     
  8. Feb 13, 2009 #7
    Re: derivative help!!!!!!!

    no i get zero if i do the quotient rule again.....????
     
  9. Feb 13, 2009 #8

    jgens

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    Gold Member

    Re: derivative help!!!!!!!

    No, you don't need to apply the quotient rule again nor should you; moreover, the derivative of 1/cosh(x)^2 is not zero.

    Maybe you haven't been taught this but sech(x) = 1/cosh(x). Now simplify it.
     
  10. Feb 13, 2009 #9
    Re: derivative help!!!!!!!

    oooo yessss i found it in my notebook in the hyperbolic functions would it be sech^2x???????????
     
  11. Feb 13, 2009 #10

    jgens

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    Gold Member

    Re: derivative help!!!!!!!

    Yes, d(tanh(x))/dx = sech(x)^2.
     
  12. Feb 14, 2009 #11

    Mark44

    Staff: Mentor

    Re: derivative help!!!!!!!

    You've come to this Web site, so I suppose that means you would like some help. You've taken the first step, which is to show us what you're tried to do. Good.

    One thing you can do to help your cause is to provide meaningful information.
    "find f'(x) sinhx/coshx"
    Given just this information, I wouldn't know what to do. You're asking us to find the derivative of some function, but you haven't explicitly told us what the function is. Of course, we might infer that f(x) = sinhx/coshx, and you want to find f'(x).
    It's harder to write mathematical expressions on a single line than on paper, where you can write fractions more easily. When you have to write a complicated expression with fractions on a single line, make sure that you put a pair of parentheses around the entire numerator and another around the entire denominator. For example, if I write 3 + 1/4, most anyone would interpret this as 3 1/4, not as 4/4 = 1.
     
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