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Derivative of a trinomial

  1. Aug 9, 2007 #1
    Im don't know how to find the derivative of this trinomial:
    y=.2x(x-5.1)(x-9.1)
    I tried multiplying the 2nd and 3rd terms by the first one
    which results in (.2x^2-1.02x)(.2x^2-9.1x), then with these two terms i used the product rule:
    y'=u'v+v'u
    The answer i got was y=(.4x-1.02*.2x^2-9.1x)+(.4x-9.1*.2x^2-1.02x)
    i checked if this was correct by graphing the derivative against my derivative on my calculator and they didnt match up.
     
  2. jcsd
  3. Aug 9, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    This is incorrect. You can multiply the .2x into the 2nd or the 3rd factor, but not both! (a*b*c does not equal [a*b]*[a*c] = a^2*b*c)

    You can also just multiply the entire polynomial out to make taking the derivative even easier.
     
  4. Aug 9, 2007 #3
    What you did is wrong, you can't multiply both by .2x, only one of them.

    --EDIT--

    Sorry, I didn't know Doc Al already posted.
     
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