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Related rates shadow problem

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A spotlight on theground shines on a wall 12 m away. If a man 2m tall walks from the spotlight toward the building at a speed of 1.6m/s. how fast is the length of his shadow on the building decreasing when he is 4m from the building?

    3. The attempt at a solution

    x=4m dx/dt=1.6m/s dy/dt=?

    I am kinda of stuck on this one. I know that as x decreases z and y will also decrease. But I am really not to sure how to proceed on this one any hints?
  2. jcsd
  3. Oct 25, 2009 #2


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    You need a relation between x, the distance from the wall and y, the height of the shadow. Draw a picture and find two similar triangles.
    Last edited: Oct 25, 2009
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