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!

Differentiation problem

  1. Jan 14, 2009 #1
    Hey I'm stuck on a problem, i have to find the differential of y=5x(2x-1)^3 and therefore find the x-coordinate of a stationary point on a graph,
    I use the chain rule and get the differential to be 30x(2x-1)^2 and therefore the x-coordinate to be 0.5
    however my textbook says the differential is 5(2x-1)^2(8x-1) and the x-coordinate of the stationary point is 1/8th I was wondering if someone could explain how this answer was obtained. Thanks :confused:
  2. jcsd
  3. Jan 14, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    You used the chain rule, but you forgot about the product rule?
    If you open up the last pair of brackets the first term is precisely what you got.
  4. Jan 14, 2009 #3
    Ah thank you very much but i was always taught to do the chain rule in equations like the one above but the product rule worked :P again, thanks.
  5. Jan 14, 2009 #4


    User Avatar
    Science Advisor

    Sometimes you need both!:wink:
  6. Jan 14, 2009 #5


    User Avatar
    Science Advisor
    Homework Helper

    If you have a product of two (or more things) you always start with the product rule,
    (f(x) g(x))' = f'(x) g(x) + f(x) g'(x)

    This obviously requires you to calculate f'(x) and g'(x) and you may need the chain rule for either f'(x), or g'(x), or both. For example, for the derivative of
    (3x + 6)^2 (2x - 1)^3
    you will need the product rule once and the chain rule twice.
  7. Jan 14, 2009 #6
    I understand now. Thanks for all the help guys :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Differentiation problem
  1. A differential problem (Replies: 6)

  2. Differentiation Problem (Replies: 26)