Engineering
posted by Alexander .
4. A rectangular enclosure is formed by using 800m of fencing. Find the greatest possible area that can be enclosed in this way and the corresponding dimensions of the rectangle.

4 x = 800
x = 200 feet on a side square
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prove that square is max area
A = x y
P = 2 x + 2 y perimeter
so x + y = P/2
A = x (P/2  x) = x^2 +P/2 x
x^2  (P/2) x = A find vertex,complete square
x^2  (P/2)x + P^2/16 = A + P^2/16
(xP/4)^2 = (AP^2/16)
vertex at x= P/4 (then y = P/4 too :)
A = (P/4)^2
so square with side = Perimeter/4 is max area
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