Thread: "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]

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