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Math Help - differentiating cosh

  1. #1
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    differentiating cosh

    f(x)=ln(cosh(7x+9))
    I have to differentiate the function above. I think you should use the chain rule but it doesnt seem to be working for me. The answer is 7tanh(7x+9). But i dont know where the the tanh comes from. Can anybody help?
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  2. #2
    Super Member
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    f(x) = \ln( \cosh(7x+9))

    f'(x) =  \frac{7 \sinh(7x+9)}{\cosh(7x+9)}

    did you manage to get this far ?

    if so all you need to know is that \tanh x = \frac{\sinh x}{\cosh x}
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