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Homework Help: Need help with basic dervivative problem

  1. Dec 29, 2011 #1
    1. The problem statement, all variables and given/known data
    they want f'(x)
    of f(x)=2x^2 +x-1

    2. Relevant equations
    I know how to use the power rule and such and get that answer to be 4x+1...but I am practicing using the definition of a dervivative and I keep getting 4x-1
    ...so ya need help

    relevent equations: f'(x)= Lim h→0 of f(x+h) - f(x) all over h

    3. The attempt at a solution
    started out with
    2(x+h)^2 +(x+h) -(2x^2 +x-1) all over h

    then I got 2x^2 +2h^2 +2xh +x+h-1-2x^2 - x +1 all over h

    and cancled out and got
    2h^2 +4xh +h -1 all over h

    then I factored out the h and got
    h(2h+4x)-1 all over h

    cancled out the h and got
    2h+4x-1 all over 1

    entered in 0 for h and i got 4x-1
    so what the heck did I do wrong??? haha, I think I had my set up wrong from the beginning??? anyone willing to help me out here, just started self studying calculus so a little confused....
  2. jcsd
  3. Dec 29, 2011 #2


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    Homework Helper

    This is wrong. It should be
    [tex]\frac{2(x + h)^2 + (x + h) \textbf{ - 1} - (2x^2 + x - 1)}{h}[/tex]
    This isn't right either. This should be
    [tex]\frac{2x^2 + \textbf{4xh} + 2h^2 + x + h - 1 - 2x^2 - x + 1}{h}[/tex]
    Nope. This should be
    [tex]\frac{4xh + 2h^2 + h}{h}[/tex]
    You should be able to figure out the rest from here.
    Last edited: Dec 29, 2011
  4. Dec 29, 2011 #3
    right i got it....wrong set up, thank you very much :) now I see how to set those up haha, thank you.
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