1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Derivative help *upgraded*

  1. Mar 6, 2013 #1
    Sorry for not showing my work on my last post, I'm on my good for nothing blackberry and apparently cannot type. My question was:
    Use first principles definition to find dy/dx:
    x--> 0

    (X+h, -4/3(X+h)

    =lim»0 ((-4/3(x+h))-(-4/3x))/x+h-x

    I have gotten the answer 4/3x^2 through my proof, but I knew what the answer was (I looked at the back in frustration). I'm afraid I may have broken a few rules when I found a common denominator for the numerator ( (3)(x+h)(3x)) and worked it down to

    =lim»0 -12x+12x+12h/[3x+3h](3x)

    =lim »0 4h/3x^2h+3hx

    Any help is appreciated !!!!
    Last edited: Mar 6, 2013
  2. jcsd
  3. Mar 6, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper

    That's still pretty hard to read. You are leaving out too many parentheses. If you mean y=(-4)/(3x) then if you got (4h)/(3x^2+3xh) for the numerator then it's ok. Now the denominator is h, so divide by that and let h->0. 4/(3x^2) is correct.
    Last edited: Mar 6, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted