Question

A rancher has 600600 feet of fencing to put around a rectangular field and then subdivide the field into 33 identical smaller rectangular plots by placing two fences parallel to one of the field's shorter sides. Find the dimensions that maximize the enclosed area. Write your answers as fractions reduced to lowest terms.

Answer 1

Answer:

x =  150 ft

y  =   y  =  150 / 17 ft

Step-by-step explanation:

Area of the field is    A  =  x*y

Let´s assume x is the larger side of the rectangle, and that we will get 33 identical smaller rectangular plots dividing  x into 33 rectangles ( using 34 sides of length  y ) then the quantity of fence is:

p  =  2*x  +  34*y   ⇒   600  =  2*x  +  34*y        y  =  ( 600 - 2*x)/34

A = x*y

A (x)  =  x*  ( 600 - 2*x)/34

A (x)  = ( 600*x  - 2*x²)/34

Taking derivatives on both sides of the equation:

A´(x)  = (1/34)* ( 600 - 4*x )

A´(x)  =  0       (1/34)* ( 600 - 4*x ) = 0      600  -  4*x  = 0

x  =  600/4

x = 150 feet

And  y  =  (1/34)( 600 -  2*150)

y  =  300/34    y  =  150 / 17 ft

y  =  8.82 ft

How do we know that is a maximum area, we find the second derivative of A

A´´(x)   =    - 4/34      then  A´´(x) < 0

We get a maximum of function A for the value  x = 150