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A rectangular garden 800 square feet in area is to be fenced off against rodents. Find the dimensions that will require the least amount of fencing if one side of the garden is already protected by a barn.

Sagot :

Step-by-step explanation:

the area of a rectangle is

a × b

with a, b being the length and width (in any sequence).

a×b = 800

a = 800/b

the perimeter is

2a + 2b

one side is not needed, so we take one of them off. e.g. we take off one a.

a + 2b need to be a minimum

800/b + 2b

f(b) = 800/b + 2b

the extreme value(s) we find as the zero points of the first derivative.

f'(x) = -800/(b²) + 2 = 0

2 = 800/(b²)

2b² = 800

b² = 400

b = ±20 ft

negative lengths don't make sense, so, 20 ft is the solution.

a×b = 800

a×20 = 800

a = 40 ft

so, the minimum fencing is for

a = 40 ft

b = 20 ft (2 times)

the second a is covered by the barn.