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Homework Help: Differentiation dy/dx Problem

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data
    If y^2 - 2xy=21, then dy/dx at the point (2,-3) is ?

    2. Relevant equations
    y^2 - 2xy=21

    3. The attempt at a solution
    I know that I have to differentiate the function. I just do not know how to do it: implicitly or explicitly.
  2. jcsd
  3. Dec 16, 2008 #2


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    Science Advisor

    Why not try both ways?

    Actually, you should get the same (correct) answer doing either.)
  4. Dec 16, 2008 #3
    i tried to diff it in terms of dy/dx but I don't think i got it right. i got dy/dx to equal 2y+2x
  5. Dec 16, 2008 #4
    what I did was...
    y^2= 2xy +21
    2(dy/dx)= 2y+2x(dy/dx)
    Divide by dy/dx on both sides
  6. Dec 16, 2008 #5
    Never mind, I see what I did wrong. I should have put the two dy/dx's on the same side, take out dy/dx and then just divide to get dy/dx alone.
  7. Dec 16, 2008 #6


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    Homework Helper

    y^2= 2xy +21

    This step is wrong. It should be
    2y*dy/dx = 2(y+x(dy/dx))
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