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A farmer wants to fence in a rectangular plot in a large field, using a rock wall which is already there as the north boundary. The fencing for the east and west sides of the plot will cost $4 a yard, but the farmer needs to use special fencing which cost $5 a yard on the south side of a plot. If the area of the plot is to be 600 square yards, find the dimensions for the plot which will minimize the cost of the fencing.

Respuesta :

caylus

Hello,

Let's assume x the length of the north's wall

and y the length of west side .

Area:x*y=600  ==> y=600/x

Cost: 2*y*4+5*x=8y+5x= 4800/x+5x

Minimize cost: (4800/x+5x)'=0

-4800/x²+5=0

x²=4800/5

x²=960

[tex]x=8\sqrt{15} \\\\y=5\sqrt{15} \\[/tex]

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