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!

Homework Help: Derivative help

  1. Jan 29, 2007 #1
    1. The problem statement, all variables and given/known data
    This problem is not hard at all its just that this stupid online homework program is a problem. Anyways,


    The quotient:

    This can be simplified to:

    What is A & B
    What is f'(7)?

    2. Relevant equations


    3. The attempt at a solution

    The derivative is easy to get.
    The final form is [tex]\frac{-5}{210h+1764}[/tex]
    How do I get it to:

    Last edited: Jan 29, 2007
  2. jcsd
  3. Jan 29, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Why not multiply top and bottom by -1/5?
  4. Jan 29, 2007 #3
    but when I input B=210 and A=1764 it tells me I'm incorrect
  5. Jan 29, 2007 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Well, you haven't multiplied them by -1/5. In order to get [tex]\frac{-5}{210h+1764}[/tex] into the required form, you need to multiply top and bottom by -1/5 (to get unity in the numerator).
  6. Jan 29, 2007 #5
    Ah..of coarse.
    There is another problem that requires the form Ah^2 + B h + C.
    f(x) = 2x^2 + 9 x + 4, find f'(2).

    Now I got :
    2h+17, A is 0, B is 2 but
    when i input 17 for c it is incorrect. Why?
    Last edited: Jan 29, 2007
  7. Jan 29, 2007 #6

    Gib Z

    User Avatar
    Homework Helper

    Umm do you have to do it by first principles? It seems the quotient rule would work fine here.

    For your 2nd problem, f'(2)=17, I dont see what the problem is.
  8. Jan 29, 2007 #7
    Well the question wants the form Ah^2+Bh+C
    I know that
    Shouldnt C=17? When i input that it is wrong.
  9. Jan 29, 2007 #8
    If not c=17, I don't understand what else it could be!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook