Question

a rectangular area of 24000 square feet is to be fenced on all four sides. fencing east and west sides cost $10 per foot fencing for the other two sides cost $20 per foot. what s the cost of least expensive

Answer 1

Answer:

C(min)   =  8763.4 $

Step-by-step explanation:

Area =  24000 ft²

In a rectangle,    A =  x * y            x  is the base side and  y the height side

Cost of low and upper side   (x)  is 20 $ per foot

Cost of east and west sides  ( y)  is  10 $per ft

total cost is:

C(r)  =  2 * 20* x  +  2*  10* y

from  A =  x * y      y  =  A / x        y  =  24000 / x

And by substitution in C(r) we get:

C(x)  =   2 * 20* x  +  2*  10* 24000 / x

C(x)  =  40 * x  + 480000 / x

Tacking derivatives on both sides of the equation:

C´(x)  =  40  -  480000 / x²

C´(x)  =  0                40   -   480000/x²   = 0

40* x²  -  480000  = 0

x²  =  480000 / 40

x²  =  12000

x  =  √ 12000   =  109,54 ft

and  y  =  24000 / 109,54

y  =  219,09  ft

Chequing for second derivative

C´´(x)  =   480000 / x⁴     is always positive so we have a minimum of C at the value  x  =  109,54

Minimum cost C (min)  =  40* 109,54 +  20 * 219,09

C(min)   =  4381.6  +  4381.8

C(min)   =  8763.4 $