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: Product rule for derivatives

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data

    h(t) = √t (1 - t^2)

    2. Relevant equations

    the product rule is (first) x (derivative of the second) + (second) x (derivative of the first)

    3. The attempt at a solution

    i've been working at this for a while. the closest answer i came up with was this:

    h(t)= t^1/2 (t^1/2 - t^5/2)

    h'(t)= 1/2t^1/2 (1/2t^1/2 - 5/2 t^3/2)

    h'(t) = 1 (1 - 5 t^3/2)/2t^1/2

    h'(t) = 1 - 5t^3/2 / 2t^1/2

    (the actual solution is 1 - 5t^2 / 2t^1/2)

    if anyone could show me where i went wrong with this, it would be MUCH appreciated. thanks!
  2. jcsd
  3. Oct 15, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That first step is wrong.

    Also there is no reason to use the product rule on this problem. Do a little (very little) algebra first:

    [tex]h(t) = t^{\frac 1 2} - t^{\frac 5 2}[/tex]
  4. Oct 16, 2011 #3


    User Avatar
    Science Advisor

    In your first step you seem to think that (fg)'= f'g' which is NOT the "product rule"!

    The product rule says that (fg)'= f'g+ fg'.
  5. Oct 16, 2011 #4
    thank you!! i've finally got it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook