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Math Help - Related Rates, shadow and streetlight problem

  1. #1
    Member nautica17's Avatar
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    Related Rates, shadow and streetlight problem

    I need some help setting up this related rates problem.

    A man 6 ft tall is walking at what rate of 3 ft/s toward a streetlight 18 ft high. At what rate is his shadow length changing?
    I know this problem is about using similar triangles but I'm not quite sure where to start. Can someone just give me a basic equation to start with? I've looked at similar problems but they all do things differently. :|
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  2. #2
    Behold, the power of SARDINES!
    TheEmptySet's Avatar
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    Quote Originally Posted by nautica17 View Post
    I need some help setting up this related rates problem.

    I know this problem is about using similar triangles but I'm not quite sure where to start. Can someone just give me a basic equation to start with? I've looked at similar problems but they all do things differently. :|
    Here is a sketch of the diagram

    Related Rates, shadow and streetlight problem-math.jpg

    Using simialr triangles you get

    \frac{18}{6}=\frac{x+y}{y} \iff y=\frac{1}{2}x

    This should get you started.
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  3. #3
    Member nautica17's Avatar
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    Quote Originally Posted by TheEmptySet View Post
    Here is a sketch of the diagram

    Click image for larger version. 

Name:	math.jpg 
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ID:	15787

    Using simialr triangles you get

    \frac{18}{6}=\frac{x+y}{y} \iff y=\frac{1}{2}x

    This should get you started.
    Thank-you!
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