The area of the garden is the product of its dimensions
From the figure (see attachment), the perimeter is:
[tex]\mathbf{P = 3x + y}[/tex]
The perimeter is given as: 120.
So, we have:
[tex]\mathbf{3x + y = 120}[/tex]
Make y the subject
[tex]\mathbf{y = 120 - 3x}[/tex]
The area is then calculated as:
[tex]\mathbf{A =xy}[/tex]
Substitute [tex]\mathbf{y = 120 - 3x}[/tex]
[tex]\mathbf{A =x(120 - 3x)}[/tex]
[tex]\mathbf{A =120x- 3x^2}[/tex]
Differentiate
[tex]\mathbf{A' =120- 6x}[/tex]
Set to 0
[tex]\mathbf{120- 6x = 0}[/tex]
Rewrite as:
[tex]\mathbf{6x = 120}[/tex]
Divide through by 6
[tex]\mathbf{x = 20}[/tex]
Recall that: [tex]\mathbf{y = 120 - 3x}[/tex]
So, we have:
[tex]\mathbf{y = 120 - 3(20)}[/tex]
[tex]\mathbf{y = 120 - 60}[/tex]
[tex]\mathbf{y =60}[/tex]
The dimension that maximizes the area is 20 by 60
The area is calculated as:
[tex]\mathbf{A =xy}[/tex]
So, we have:
[tex]\mathbf{A = 20 \times60}[/tex]
[tex]\mathbf{A = 1200}[/tex]
Hence, the maximum area is 1200 square feet
Read more about maximum areas at:
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