Answer:
width = 13 ft; area = 169 ft^2
Step-by-step explanation:
The equation for the area can be factored as ...
A = x(26 -x)
This has zeros at x=0 and x=26. The vertex of this downward-opening parabola will be halfway between those, at x=13. Then the area for a width of 13 feet is ...
A = 13(26 -13) = 13^2 = 169 . . . . ft^2
The maximum area is 169 ft^2, when the width is 13 ft.
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You will note that these dimensions make the pen be a square. It can be useful to remember that a square is the rectangle with the largest area for a given perimeter length.