Answer:
Dimensions: 125 m x 250 m
Area: 31,250 m²
Step-by-step explanation:
Given information:
- Total amount of fencing = 500m
- Only 3 sides of the land need to be fenced
First, let us assume that the land is rectangular in shape.
Let [tex]y[/tex] = length of the side opposite the river
Let [tex]x[/tex] = length of the other 2 sides of the land
Therefore, we can create two equations from the given information:
Area of land: [tex]A= xy[/tex]
Perimeter of fence: [tex]2x + y = 500[/tex]
Rearrange the equation for the perimeter of the fence to make y the subject:
[tex]\begin{aligned} \implies 2x + y & = 500\\ y & = 500-2x \end{aligned}[/tex]
Substitute this into the equation for Area:
[tex]\begin{aligned}\implies A & = xy\\& = x(500-2x)\\& = 500x-2x^2 \end{aligned}[/tex]
To find the value of x that will make the area a maximum, differentiate A with respect to x:
[tex]\begin{aligned}A & =500x-2x^2\\\implies \dfrac{dA}{dx}& =500-4x\end{aligned}[/tex]
Set it to zero and solve for x:
[tex]\begin{aligned}\dfrac{dA}{dx} & =0\\ \implies 500-4x & =0 \\ x & = 125 \end{aligned}[/tex]
Substitute the found value of x into the original equation for the perimeter and solve for y:
[tex]\begin{aligned}2x + y & = 500\\\implies 2(125)+y & = 500\\250+y & = 500\\y & = 250\end{aligned}[/tex]
Therefore, the dimensions that will give Christine the maximum area are:
125 m x 250 m (where 250 m is the side opposite the river)
The maximum area is:
[tex]\begin{aligned}\implies \sf Area_{max} & = xy\\& = 125 \cdot 250\\& = 31250\: \sf m^2 \end{aligned}[/tex]
