Results 1 to 6 of 6

Math Help - derivative

  1. #1
    Junior Member
    Joined
    Sep 2009
    Posts
    56

    derivative

    f(x)=(x^5-4/x)^-4

    f'(x)=

    gave me -4(5x^4-4)/(x^5-4/x)^5 , but its wrong
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor harish21's Avatar
    Joined
    Feb 2010
    From
    Dirty South
    Posts
    1,036
    Thanks
    10
    Quote Originally Posted by wannous View Post
    f(x)=(x^5-4/x)^-4

    f'(x)=

    gave me -4(5x^4-4)/(x^5-4/x)^5 , but its wrong
    Is your question [\frac{x^{5} - 4}{x}]^{-4} ?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Sep 2009
    Posts
    56
    only the -4 is divided by x
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Senior Member
    Joined
    Jan 2010
    Posts
    354
    So it is:

    f(x)=\left( x^5 - \frac{4}{x} \right) ^{-4}

    It looks like your derivative of x^5 - \frac{4}{x} was wrong.

    \frac{d}{dx} \left( x^5 - \frac{4}{x}\right) = 5x^4 + \frac{4}{x^2}
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Junior Member
    Joined
    Sep 2009
    Posts
    56
    aww so i needed to use quotient rule on the 4/x
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Senior Member
    Joined
    Jan 2010
    Posts
    354
    Quote Originally Posted by wannous View Post
    aww so i needed to use quotient rule on the 4/x
    That does work, but you can also do it by rewriting it first as 4x^{-1} then using the power rule, which gives you -4x^{-2}, then changing it back to a fraction to get -\frac{4}{x^2}.

    By the way, in general, you'll probably want to just commit to memory that the derivative of \frac{1}{x} is -\frac{1}{x^2} since it seems to come up often enough that it will be a time saver to just learn it.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. contuous weak derivative $\Rightarrow$ classic derivative ?
    Posted in the Differential Geometry Forum
    Replies: 2
    Last Post: April 22nd 2011, 03:37 AM
  2. Replies: 0
    Last Post: January 24th 2011, 12:40 PM
  3. [SOLVED] Definition of Derivative/Alt. form of the derivative
    Posted in the Calculus Forum
    Replies: 2
    Last Post: September 23rd 2010, 07:33 AM
  4. Derivative of arctan in a partial derivative
    Posted in the Calculus Forum
    Replies: 3
    Last Post: September 12th 2010, 02:52 PM
  5. Replies: 2
    Last Post: November 6th 2009, 03:51 PM

Search Tags


/mathhelpforum @mathhelpforum