Question

If the garden is to be 1250 square feet, and the fence along the driveway costs $6 per foot while on the other three sides it costs only $2 per foot, find the dimensions that will minimize the cost.

Answer 1

Answer:

Dimensions of rectangular garden:

x = 25 feet   ( sides along the driveway)

y = 50 feet

Step-by-step explanation:

Rectangular area is:

A(r)  = x*y           (1)

if we call x one the driveway side the cost of that side will be

6*x

The cost of the other side parallel to driveway side is 2*x and cost of the others two sides are 4*y

Total costs:  C = 6*x + 2*x  * 4*y     (2)

From equation (1)

A(r)  = 1250 = x*y      ⇒⇒   y = 1250/ x

Plugging that value in equation (2) we get costs as a function of x

that is:

C(x) = 6*x + 2*x +  4* 1250/x

Taking derivatives on both sides of the equation

C´(x)  = 6 + 2 - 5000/x²

C´(x)  = 8 - 5000 /x²

C´(x) = 0       ⇒       8 - 5000 /x² = 0

8*x² -5000 = 0

x² = 5000/8

x² = 625

x = 25 feet

and    y = 1250/ 25

y = 50 ft

C(min) = 50*2*2 + 6*25 + 2*25

C(min) = 200 + 200

C(min) = 400 $