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Math Help - Dimensions

  1. #1
    Member >_<SHY_GUY>_<'s Avatar
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    Lightbulb Dimensions

    I have a few quick questions for this project i have...
    one of them ask:

    5. if you wanted the area to be 20 m^2, what would the dimensions of a rectangle to minimize the amount of fencing needed? use a graphing calculator to determine the answer.

    -->i remember him saying that the the dimensions is related to the square root of 20. i just don't remember how to get the formula to put it in the calculator and show it...


    the whole project's purpose is to show him our knowledge to solve real-life problems that can be analyzed by Graphical, Numerical or Algebraic. [Numerical meaning completing a table of values]

    and one question asks: what are the weaknesses and strengths of these strategies?

    any help would be appreciated...thank you!
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  2. #2
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    Let x and y be the dimensions of the rectangle
    Area is xy=20
    Perimeter is P=2(x+y)=2(x+20/x)
    Differentiate P gives P'(x)=2(1-20/x)
    P'(x)=0 for x=square root of 20
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  3. #3
    Member >_<SHY_GUY>_<'s Avatar
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    Quote Originally Posted by running-gag View Post
    Let x and y be the dimensions of the rectangle
    Area is xy=20
    Perimeter is P=2(x+y)=2(x+20/x)
    Differentiate P gives P'(x)=2(1-20/x)
    P'(x)=0 for x=square root of 20
    thats where i got lost...how did you get x^2 in your denominator?
    im trying to work out the problem but i know im doing something wrong
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  4. #4
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    Remember that differentiate 1/x gives -1/x
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  5. #5
    Member >_<SHY_GUY>_<'s Avatar
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    Quote Originally Posted by running-gag View Post
    Remember that differentiate 1/x gives -1/x
    Oh... Sorry!
    Thank you!
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