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Homework Help: Law of Cosines and Related Rates

  1. Apr 19, 2006 #1
    Here is the question:
    Two people start from the same point. One walks east at 3 mi/h and the other walks northeast at 2 mi/h. How fast is the distance between the people changing after 15 minutes?
    I have:
    dx/dt= 3 mi/h, dy/dt= 2 mi/h, dz/dt= ?
    x= 3*.25= .75, y= 2*.25= .50
    The instructor hint was to use the Law of Cosines:
    [tex]z^2 = x^2 + y^2 - 2xy \cos \theta[/tex]
    My theta is 45 degrees.
    My question is that I am plugging in values for all the variables, but I think this is the wrong direction.
    Thanks for your help.
    Last edited: Apr 19, 2006
  2. jcsd
  3. Apr 20, 2006 #2
    Why do you think it is the wrong direction?
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