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 of a fraction

  1. Aug 26, 2007 #1
    Hi
    Im trying to find the derivative of y=9/x+5
    I tried using the quotient rule and
    and the derivative of (x+5)^-1*9 but both methods dont work.
     
  2. jcsd
  3. Aug 26, 2007 #2
    If you would show us some work, someone could point out the problem you are having.
     
  4. Aug 26, 2007 #3

    hotvette

    User Avatar
    Homework Helper

    Do you mean y=(9/x)+5 or y=9/(x+5)? From your post, I presume the latter.

    As d_leet said, show us your work and we'll guide you.
     
    Last edited: Aug 26, 2007
  5. Aug 26, 2007 #4
    its ok i found what i was doing wrong. I made a mistake with the quotient rule. i had u'v+v'u instead of u'v-v'u
     
  6. Aug 26, 2007 #5

    hotvette

    User Avatar
    Homework Helper

    By the way, I'm curious as to why you are using the quotient rule, unless it was a requirement for this problem. Normally the quotient rule is used in the following situation:

    [tex]y = \frac{g(x)}{h(x)}[/tex]

    In your case, g(x) is a constant and y' can be found directly. Let me ask this. How would you find y' if y = 1/x?

    As a side note, I never use the quotient rule, because I can never remember how it works. I always use the product rule:

    [tex]y = g(x)h(x)^{-1}[/tex]
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook