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Math Help - "Primary", "Secondary" equation

  1. #1
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    "Primary", "Secondary" equation

    A fence is to be built to enclose a rectangular region of 4800 square feet. The fencing material along three sides costs $3 per foot.The fencing material along the fourth side costs $4 per foot.

    Find the most economical dimensions of the region. In addition to answering the question, label your “primary” equation, “secondary” equation.

    ---Could someone help me figure out how my professor got these primary & secondary equations


    [primary: C = 6y + 7x ; secondary: xy=4800]
    [answer: 64.14 ft by 74.84 ft]
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  2. #2
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    You will have two equations: one for the area and another for the cost.
    The secondary equation gives you the are
    Lenght times Width = Area , that is : xy = 4800 sqr. feet.
    and the primary is the cost:
    C = 3$ for (2 equal sides X + 1 side Y) + 4$ for (1 side Y)
    You write this as follows:
    C = 3(2x+y) + 4(y)
    C= 6x + 3y +4y
    C = 6x + 7y
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