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Homework Help: Finding rate of change of shadow problem.

  1. Feb 19, 2009 #1
    Question: A 1.85 m tall man is walking toward a 12 m tall street light at night at a rate of 2.2 m/s. How fast is the length of his shadow changing when he is 12 m from the street light?

    so, using similar triangles, i got that (x+y)/12 = x/1.85. I can rearrange this into x= or y=, but then I don't see how i can find the derivative of it (ex. x = (1.85x+1.85y)/12 -> finding derivative will eliminate x and y). Any help will be greatly appreciated.

    Thanks!
     
  2. jcsd
  3. Feb 19, 2009 #2

    Dick

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    The derivative of that is (dx/dt+dy/dt)/12=dx/dt/1.85. The problem statement tells you what one of those derivatives is. Which one and what's it's value?
     
  4. Feb 19, 2009 #3
    Oh I think I got it. So just to make sure: I solve for dx/dt plugging in -2.2 for dy/dt correct?
     
  5. Feb 19, 2009 #4

    Dick

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    Exactly.
     
  6. Feb 19, 2009 #5
    Thanks alot Dick!
     
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