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Homework Help: Related Rates Question. Help!

  1. Oct 14, 2006 #1
    Hey, I have this one practise calculus question that I just can't seem to get. Any help would be greatly appreciated:

    A light is on the ground 40ft from a building. A man 6ft tall walks from the light towards the building at 6ft/s. How rapidly is his shadow on the building becoming shorter when he is 20ft from the building?
    (btw, the answer should be -3.6ft/s)

    I've already drawn a diagram, but I'm not sure if triangle ratios would do any good.
  2. jcsd
  3. Oct 14, 2006 #2


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    Were you able to find the height of his shadow as a function of his distance from the building?
  4. Oct 14, 2006 #3
    First draw a picture. You know its going to be a triangle. Let x be the distance the man is from the light. And let y be the height of his shadow on the building. You know dx/dt = 6. We also know that the length of the triangle is 40 ft. So we can divide the side into x + (40-x). So how would you use similar triangles to find y as a function of x?

    Hint: [tex] \frac{6}{x} = \frac{y}{?} [/tex]
    Last edited: Oct 14, 2006
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