a farmer wants to construct a fence around an area of 150 square feet in a rectangular field and then divide it in half with a fence parallel to one of the sides of the rectangle. what dimensions should the fenced area have in order to minimize the length of fencing used?



Answer :

The lengths of the sides of the rectangular field so as to minimize the amount of fencing needed is 15 feet by 10 feet.

Let x represent the length of the fence and y represent the width of the fence.

Since the area is 150, hence:

Area = length × width = x × y

150 = xy

y = 150/x

A fence divide the field in half and is parallel to one of the sides of the rectangle. Hence:

Amount of fencing needed (P) = x + x + y + y  + y = 2x + 3y

Amount of fencing needed (P) = 2x + 3(150/x) = 2x + 450/x

To minimize the amount of fencing needed, dP/dx = 0, hence:

dP/dx = 2 - 450/x²

2 - 450/x² = 0

2 = 450/x²

2x² = 450

x² = 225

x = 15 feet

Let's find the value of y.

y = 1350/x = 1350/15 = 10 feet

Hence, the lengths of the sides of the rectangular field so as to minimize the amount of fencing needed is 15 feet by 10 feet.

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