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Derivative help

  1. Apr 8, 2015 #1
    1. The problem statement, all variables and given/known data

    I have to derivate f(x)

    f(x) = 1/x +1/(x+1)

    Answer is = -1/x2 - 1/(x+1)2
    I cant seem to get to that answer :(

    thank you
    2. Relevant equations


    3. The attempt at a solution
    f(x) = (1/x +1/(x+1))'
    = (2x+1)/x(x+1)' = ((2x+1)' * (x(x+1)-(2x+1)*(x(x+1)')/x2(x+1)2
    =(2(x(x+1)-(2x+1)*(x(1)+(x+1))/x2(x+1)2
    =(2x(x+1)-(2x+1)*(2x+1))/x2(x+1)2
    =2(x(x+1))-(2x+1)2/x2(x+1)2
     
    Last edited: Apr 8, 2015
  2. jcsd
  3. Apr 8, 2015 #2
    It seems to me that you are doing a bunch of extra work. You don't need to combine the terms over a common denominator to evaluate the derivative because the derivative of a sum is simply the sum of the derivatives.

    Differentiate each term separately. If it helps, rewrite the terms with exponents instead of having them as fractions (i.e. write 1/x as x-1)
     
  4. Apr 8, 2015 #3

    joshmccraney

    User Avatar
    Gold Member

    Try re-writing ##f(x)## as ##f(x)=x^{-1} + (x+1)^{-1}##. From here you can use the chain rule on the second term.
     
  5. Apr 8, 2015 #4
    You actually didn't post a question but from the answer given, the function was differenciated, which is calculus and therefore shouldn't be posted here.
     
  6. Apr 8, 2015 #5

    Mark44

    Staff: Mentor

    @helppleasemath, minor point, but there is no such word in English as "derivate" at least in the context of calculus. The word you want is "differentiate", the action you perform to get the derivative.
     
  7. Apr 8, 2015 #6
    oh ok sorry wasnt sure what word to use lol.
     
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