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Homework Help: How fast is the distance between the cars changing at this moment

  1. Mar 3, 2010 #1
    [Solved]How fast is the distance between the cars changing at this moment

    f37rfc.jpg

    I'm in need of help with the last part (finding dz/dt).

    I'm sorry I don't know how to use latex.
    Let D refer to partial derivatives.

    This was my attempt best try:

    Given dz/dt = Dz/Dt*dx/d+ Dz/Dt*dy/dt and z = (x^2+y^2)^(1/2),

    2z*dz/dt = 2x*dx/dt + 2y*dy/dt
    2(.36+.49)^(1/2)*dz/dt= 2(.6)(-55)+2(.7)(-60)
    dz/dt = (2(.6)(-55)+2(.7)(-60)) / (2(.36+.49)^(1/2))
    dz/dt = -81.3489

    Any help would be great,
    thanks
     
    Last edited: Mar 4, 2010
  2. jcsd
  3. Mar 3, 2010 #2

    Dick

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    Homework Helper

    I don't see anything wrong with how you are working it out. The only thing that comes to mind is that you are saying the distance is "decreasing" at the rate of "-81.3489". Isn't that sort of double negative? I would say it's decreasing at the rate of +81.3489.
     
  4. Mar 4, 2010 #3
    Good call. Thanks Dick!
     
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