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The fence around Wayne Manor is going to be replaced. No fence will be required on the side lying along Gotham River. If the new wrought iron fence costs $12 per meter for the side parallel to the river, and $4 per meter for the other two sides, find the dimensions of the maximum area that can be enclosed by the fence if Bruce Wayne cannot spend more than $3600.

Sagot :

Step-by-step explanation:

Let the side parallel to the river be x meter and the other two sides be of y meter, then

 [tex]12 x+(2 y)(4)=3600[/tex]

[tex]12 x+8 y &=3600[/tex]

[tex]3 x+2 y &=900[/tex]

[tex]2 y &=900-3 x[/tex]

 

The area of the rectangle is [tex]A=xy [/tex]

Substitute into to express the area in a single variable x as,

[tex]A(x) &=x\left[\frac{1}{2}(900-3 x)\right][/tex]

[tex]=\frac{1}{2}\left(900 x-3 x^{2}\right)[/tex]

Differentiate A(x) with respect to x and equate to zero as,

[tex]A^{\prime}(x) &=0[/tex]

[tex]\frac{d}{d x}\left[\frac{1}{2}\left(900 x-3 x^{2}\right)\right]=0[/tex]

[tex]900-3(2 x) =0[/tex]

x=150

Here, [tex]A^{\prime \prime}(x)=-3 x<0[/tex] at [tex]x=150 \Rightarrow A[/tex] is maximum

Therefore, the dimension is:

Length of side parallel to the river: x=150 m.

Length of other two side: [tex]y=\frac{1}{2}(900-450)=225[/tex] m.

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