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Homework Help: Question about using derivatives

  1. Apr 10, 2012 #1
    1. The problem statement, all variables and given/known data
    Find the approximate percentage changes in the given function y = f(x) that will result from an increase of 2%
    y = x2

    2. Relevant equations

    3. The attempt at a solution
    dy/dt = dx/dt * d/dx * x2
    dy/dt = dx/dt * 2x
    dy/dt = 2/100 * 2x
    dy/dt = 4x% ? I don't know if I did this right but apparently in the back of the book I'm not suppose to have the x in there, why not?
  2. jcsd
  3. Apr 10, 2012 #2
    Would it be because your supposed to take the second derivative of it? That would make it just 2. I think as the question wants no variables in it too.
  4. Apr 10, 2012 #3
    I don't think that is correct...
  5. Apr 10, 2012 #4


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    Gold Member

    What is 't'? You have [itex]y = x^2[/itex] and x seems to have no dependence on a parameter 't'. You simply need to determine [itex]{{dy}\over{dx}}[/itex]

    And no, there are no second derivatives involved.
  6. Apr 10, 2012 #5
    Do you mind filling me in a bit more, i.e. is my working wrong apart from that
    Last edited: Apr 10, 2012
  7. Apr 10, 2012 #6
    There is not really a working apart from that. So no, it's not correct.
  8. Apr 10, 2012 #7
    dy/y=2x dx/x2
    dy/y=2 * 2/100
    dy/y = 4%
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