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Homework Help: Derivative arctan function Please help

  1. Jan 28, 2008 #1
    Derivative arctan function!!! Please help!!

    Find the derivative of tan^(-1)(sqrt(8x^2-1)))

    I knwo that the derivative of tan^(-1)(x) is 1/(1+x^2) and that you are supposed to use the chain rule twice but i cannot seem to get the right answer.
    If some one could please show me the steps i could figure out where i went wrong! Thanks!!!
  2. jcsd
  3. Jan 28, 2008 #2


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    Show us what you did first. The first chain rule involves the tan^(-1) and the second involves the sqrt.
  4. Jan 28, 2008 #3
    okay what i did was

    1. f(x) = arctan((sqrt(8x^2-1)))

    2. f'(x) = ((1/(1+(8x^2-1))) * (1/2)(8x^2-1)^(-1/2) * 16x

    3. = 16x/((8x^2) * 2 * sqrt(8x^2-1))

    4. = 1/(x(sqrt(8x^2-1))
  5. Jan 28, 2008 #4


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    I believe that is correct.
  6. Jan 28, 2008 #5
    really???!!! That would explain why i was so confused. These online assignments are no fun. I guess i just have to work on submitting my answers properly. Ill just double check (again).
    and thank you so much!!!!! I have been tearing my hair out over this for about an hour now trying to figure out where i went wrong.
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