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Homework Help: Implicit Differntation

  1. Apr 11, 2009 #1
    I've been working on this problem for a while and can't see my mistake, in the book the answer is stated as being 3, but I end up getting 13.5.

    1. The problem statement, all variables and given/known data
    If x^2+3y^2+2y = 10; dx/dt = 2, x=3 and y = -1 find dy/dt

    3. The attempt at a solution
    d/dt[x^2+3y^2+2y] = d/dt[10]

    3x^2 (dx/dt) + 6y(dy/dt) + 2 (dy/dt) = 0

    6y (dy/dt) + 2 (dy/dt) = -3x^2 (dx/dt)

    dy/dt (6y+2) = -3x^2 (dx/dt)

    dy/dt = -3x^2 (dx/dt) / 6x+2

    dy /dt = -3(3^2)(2) / 6(3)+2 = 13.5
  2. jcsd
  3. Apr 11, 2009 #2


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    Hi Iron_Brute! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    erm :redface:

    either it's x3 in the question or it's 2x in the derivative :wink:
  4. Apr 11, 2009 #3
    Ugh I can't believe it, it's x^2 in the text but I wrote x^3 on my sheet. Thanks for the help (and the tip on writing the exponents didn't know that), I can't believe I spent so much time looking my problem over and didn't see that. :blushing:
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