Question

A fence is to be built to enclose a rectangular area of 320 square feet. The fence along three sides is to be made of material that costs 6 dollars per foot, and the material for the fourth side costs 13 dollars per foot. Find the dimensions of the enclosure that is most economical to construct

Answer 1

The dimensions of the enclosure that is most economical to construct are; x = 14.22 ft and y = 22.5 ft

How to maximize area?

Let the length of the rectangular area be x feet

Let the width of the area = y feet

Area of the rectangle = xy square feet

Or xy = 320 square feet

y = 320/x  -----(1)

Cost to fence the three sides = $6 per foot

Therefore cost to fence one length and two width of the rectangular area

= 6(x + 2y)

Similarly cost to fence the fourth side = $13 per foot

So, the cost of the remaining length = 13x

Total cost to fence = 6(x + 2y) + 13x

Cost (C) = 6(x + 2y) + 13x

C = 6x + 12y + 13x

C  = 19x + 12y

From equation (1),

C = 19x + 12(320/x)

C' = 19 - 3840/x²

At C' = 0, we have;

19 - 3840/x² = 0

19 = 3840/x²

19x² = 3840

x² = 3840/19

x = √(3840/19)

x = 14.22 ft

y = 320/14.22

y = 22.5 ft

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