Question

A rectangular garden of area 208 square feet is to be surrounded on three sides by a brick wall costing $ 8 per foot and on one side by a fence costing $ 5 per foot. Find the dimensions of the garden such that the cost of the materials is minimized.

Answer 1

Answer:

Therefore the dimensions of the garden is 16 feet by 13 feet.

Step-by-step explanation:

Let the length of the garden be x and the width of the garden be y.

Given that the area of the rectangular garden is 208 square feet.

Therefore,

xy =208

$\Rightarrow y = \frac{208}{x}$

Again given that,

The garden is to be surrounded on three sides by a brick wall costing $ 8 per feet and the remaining sides by a fence costing $5 per feet.

The perimeter of the rectangle is = 2(length+ breadth)

                                                       = 2(x+y)

                                                       =2x+2y

The total cost of fence

$C= [ (2x\times 8)+(y\times 8)+(y\times 5)]$

   = (16x+ 8y +5y)

   $=16x+13y$

  $=16x+\frac{13\times 208}{x}$

 $=16x +\frac{2704}{x}$

To find the maximum or minimum point, we need to find out $\frac{dC}{dx}$ and set $\frac{dC}{dx}$ =0.

$\frac{dC}{dx}=16 -\frac{2704}{x^2}$

Then

$16 -\frac{2704}{x^2}=0$

$\Rightarrow \frac{2704}{x^2}=16$

$\Rightarrow x^2 =\frac{2704}{16}$

$\Rightarrow x=\pm \sqrt{169}$

$\Rightarrow x=\pm 13$

Again $\frac{d^2C}{dx}= \frac {8112}{x^3}$

$\therefore\left| \frac{d^2C}{dx}\right |_{x=13}= \frac {8112}{13^3}>0$

Therefore at x= 13 , the cost is minimum.

Therefore x = 13 feet.

The other side of the garden is $=\frac{208}{13}$ feet = 16 feet.

Therefore the dimensions of the garden is 16 feet by 13 feet.