A farmer wants to fence a rectangular area of 72 square feet next to a river. Find the length and width of the rectangle which uses the least amount of fencing if no fencing is needed along the river. Assume the length of the fence runs parallel to the river.

Step-by-step explanation:

The least fence is used when half the total fence is parallel to the river. That is, the shape of the rectangle is twice as long as it is wide.

72 = W(2W)

36 = W²

6 = W . . . . . . the width perpendicular to the river

12 = 2W . . . .  the length parallel to the river

Development of this relation

Let T represent the total length of the fence for some area A. Then if x is the length along the river, the width is y=(T-x)/2, and the area is ...

  A = xy = x(T -x)/2

Note that the equation for area is that of a parabola with zeros at x=0 and at x=T. That is, for some fence length T, the area will be a maximum at the vertex of this parabola. That vertex is located halfway between the zeros, at ...

 x = (0 +T)/2 = T/2

The corresponding area width (y) is ...

  y = (T -T/2)/2 = T/4

Equivalently, the fence length T will be a minimum for some area A when x=T/2 and y=T/4. This is the result we used above.

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