Results 1 to 2 of 2

Math Help - Trouble finding basic derivatives

  1. #1
    Member
    Joined
    Oct 2008
    Posts
    242

    Trouble finding basic derivatives

    I understand the formula to find derivatives, but for some reason I can't seem to get the right answer when applying said formula most of the time. Here's an example, maybe someone could walk me through step by step?

    3x^2-x^3

    Now I used an online derivative calculator and got the answer, but I still can't even come to that answer when I try to apply the lim (h--0) f(x+h)-f(x) over H formula. I keep getting -3x^2+2x and the correct answer is 6x-3x^2
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Member
    Joined
    Oct 2008
    Posts
    77
    Thanks
    1
    Ill get you started..

    So f(x) = 3x^2 - x^3

    Now to compute the derivative we put f(x) into the definition of the derivative.
    that is:
     \lim_{ h\to 0}  \frac{ f(x+h) - f(x)}{h} (the definition of a derivative)

    Now.. ( f(x)= 3x^2-x^3 into the definition)
     \lim_{ h\to 0}  \frac{ 3(x+h)^2-(x+h)^3 - (3x^2 - x^3) }{h}

    After expanding we get:

    =  \lim_{ h\to 0}  \frac{3(x^2+2hx+h^3)-(x^3+3x^2h+3xh^2+h^3) - 3x^2 +x^3}{h}

    Canceling terms.. and to the end of the problem...

    =  \lim_{ h\to 0}  \frac{6hx+3h^2-3x^2h-3xh^2-h^3}{h}

    =  \lim_{ h\to 0}  6x+3h-3x^2-3xh-h^2

    =  6x-3x^2
    Last edited by Scopur; October 16th 2008 at 04:43 PM.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] basic topology trouble
    Posted in the Differential Geometry Forum
    Replies: 5
    Last Post: August 5th 2011, 12:53 PM
  2. Having trouble with basic Derivatives
    Posted in the Calculus Forum
    Replies: 1
    Last Post: September 8th 2010, 06:33 PM
  3. Trouble with Derivatives
    Posted in the Calculus Forum
    Replies: 8
    Last Post: May 10th 2010, 07:48 AM
  4. Replies: 1
    Last Post: January 30th 2009, 09:02 AM
  5. Trouble with basic logrithmic functions...
    Posted in the Pre-Calculus Forum
    Replies: 8
    Last Post: November 2nd 2008, 12:13 PM

Search Tags


/mathhelpforum @mathhelpforum