Answer:
width = 1.77845 ft
length = 7.1138 ft
height = 5.691 ft
total cost = $759.09
Step-by-step explanation:
volume = length x width x height
length = 4 width
base of the rectangle = 4 x 1 w = 4w²
72 (volume) = 4w²h
18 = w²h
f(c) = (2 x 4w² x $10) + (2 x wh x $5) + (2 x 4wh x $5)
f(c) = 80w² + 10wh + 40wh
f(c) = 80w² + 50wh
since we need only 1 variable (width), we have to substitute h:
h = 18/w²
f(c) = 80w² + 50w(18/w²) = 80w² + 900/w
f'(c) = 160w + 900/w²
0 = (160w³ + 900)/w²
0 = 160w³ + 900
0 = 20(8w³ + 45)
0 = 8w³ + 45
8w³ = -45
w³ = -45/8
w = ∛-45/8 = -1.77845 and 1.77845
since w cannot be negative, then it must be 1.77845
