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.
