Farmer Ed has

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meters of​ fencing, and wants to enclose a rectangular plot that borders on a river. If Farmer Ed does not fence the side along the​ river, what is the largest area that can be​ enclosed?

3 answers

What is this?

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if the side parallel to the river is x, and the other two sides are y, you have

x+2y = 9000
the area
a = xy = (9000-2y)(y) = 9000y-2y^2
The vertex of this parabola is where you have maximum area. That is at y = 9000/4

You have y, so you can get x, and can find the area.
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