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Related Rate

  1. Mar 10, 2009 #1
    1. The problem statement, all variables and given/known data
    A street light is on top of a 20-foot pole. A person who is 5 feet tall walks away from the pole at the rate of 5 feet per second. At what rate is the tip of the person's shadow moving away from the pole when he is 20 feet from the pole?

    2. The attempt at a solution

    20/5 = y/y-x (similar triangles)
    y = 4x/3 (to solve for x)

    im stuck here...

    i also checked another solution i got here but i dont know how he got dy/dt = (6/9)*5
    https://www.physicsforums.com/showthread.php?t=114901
     
  2. jcsd
  3. Mar 10, 2009 #2

    lanedance

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    Homework Helper

    well that has differnt numbers for a start...

    you have
    x = 4y/3

    what do you want to know? what is the rate of change of the length of shadow (y') in terms of the rate of change of the person distances from the pole (x')

    so differntiate both sides of your expression w.r.t t
     
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