Answer:
The dimensions of the garden that minimize the cost is 9.018 feet(length) and 13.528 feet(width)
Step-by-step explanation:
Let the length of garden be x
Let the breadth of garden be y
Area of Rectangular garden = ![Length \times Breadth = xy](https://tex.z-dn.net/?f=Length%20%5Ctimes%20Breadth%20%3D%20xy)
We are given that the area of the garden is 122 square feet
So,
---A
A landscape architect wished to enclose a rectangular garden on one side by a brick wall costing $20/ft
So, cost of brick along length x = 20 x
On the other three sides by a metal fence costing $10/ft.
So, Other three side s = x+2y
So, cost of brick along the other three sides= 10(x+2y)
So, Total cost = 20x+10(x+2y)=20x+10x+20y=30x+20y
Total cost = 30x+20y
Substitute the value of y from A
Total cost = ![30x+20(\frac{122}{x})](https://tex.z-dn.net/?f=30x%2B20%28%5Cfrac%7B122%7D%7Bx%7D%29)
Total cost = ![\frac{2440}{x}+30x](https://tex.z-dn.net/?f=%5Cfrac%7B2440%7D%7Bx%7D%2B30x)
Now take the derivative to minimize the cost
![f(x)=\frac{2440}{x}+30x](https://tex.z-dn.net/?f=f%28x%29%3D%5Cfrac%7B2440%7D%7Bx%7D%2B30x)
![f'(x)=-\frac{2440}{x^2}+30](https://tex.z-dn.net/?f=f%27%28x%29%3D-%5Cfrac%7B2440%7D%7Bx%5E2%7D%2B30)
Equate it equal to 0
![0=-\frac{2440}{x^2}+30](https://tex.z-dn.net/?f=0%3D-%5Cfrac%7B2440%7D%7Bx%5E2%7D%2B30)
![\frac{2440}{x^2}=30](https://tex.z-dn.net/?f=%5Cfrac%7B2440%7D%7Bx%5E2%7D%3D30)
![\sqrt{\frac{2440}{30}}=x](https://tex.z-dn.net/?f=%5Csqrt%7B%5Cfrac%7B2440%7D%7B30%7D%7D%3Dx)
![9.018 =x](https://tex.z-dn.net/?f=9.018%20%3Dx)
Now check whether it is minimum or not
take second derivative
![f'(x)=-\frac{2440}{x^2}+30](https://tex.z-dn.net/?f=f%27%28x%29%3D-%5Cfrac%7B2440%7D%7Bx%5E2%7D%2B30)
![f''(x)=-(-2)\frac{2440}{x^3}](https://tex.z-dn.net/?f=f%27%27%28x%29%3D-%28-2%29%5Cfrac%7B2440%7D%7Bx%5E3%7D)
Substitute the value of x
![f''(x)=-(-2)\frac{2440}{(9.018)^3}](https://tex.z-dn.net/?f=f%27%27%28x%29%3D-%28-2%29%5Cfrac%7B2440%7D%7B%289.018%29%5E3%7D)
![f''(x)=6.6540](https://tex.z-dn.net/?f=f%27%27%28x%29%3D6.6540)
Since it is positive ,So the x is minimum
Now find y
Substitute the value of x in A
Hence the dimensions of the garden that minimize the cost is 9.018 feet(length) and 13.528 feet(width)