Answer:
dc/dx = 0 when x = 20. hence fence is 20×40
Step-by-step explanation:
Given data:
Rectangular Area = 800 ft^2
three side material cost = $5 per ft
fourth side material cost is $15 per ft
let expensive side is x and other dimension assume to be y
Then cost is c
c = 5(x+ 2y) + 15 x
xy = 800, so y = 800/x
c =5(x+ 1600/x^2) + 15x
for minimum when dc/dx = 0
(2x-40)(2x+40) = 0
x = 20
dc/dx = 0 when x = 20. hence fence is 20×40
