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Homework Help: Related Rates

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    Suppose that Y=2((x^2)-3x) and dx/dt = 2
    Find dy/dt when x=3

    2. Relevant equations
    the only calc is taking the derivative of the equation, i am wondering if i am doing the whole problem right.

    3. The attempt at a solution
    dy/dx = 4x-6
    find the equation for x=3 and multiply in 2 for the rate of change of time

    I think my answer goes back to the clac. I don't know what number i am supposed to plug in for X, i know it has to do when what X equals at dy/dt and the rate of chage of X over time.
  2. jcsd
  3. Oct 4, 2007 #2


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    Homework Helper

    You found
    (The derivative with respect to [itex]x[/itex] when [itex]x=3[/itex].)

    The problem is asking for
    (The derivative with respect to [itex]t[/itex].)
  4. Oct 5, 2007 #3


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    The problem SAYS "Find dy/dt when x= 3"!! What value of x do you think you should put in? The phrase "what x equals at dy/dt" is meaningless.
  5. Oct 5, 2007 #4
    i understand that 3 must be replaced for x, but where does the dx/dt = 2 fit into the equation? I am guessing around somewhere around the radius portion of the equation
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