Finding dimensions for a rectangle to give biggest possible area USING derivatives?

Here's the precise question: "Find the dimensions of the rectangular garden of greatest area that can be fenced off (all four sides) with 400 meters of fencing. Assume that the length of each side is between 1 and 199 meters. It's from a chapter in my calculus book called "Application of... show more Here's the precise question: "Find the dimensions of the rectangular garden of greatest area that can be fenced off (all four sides) with 400 meters of fencing. Assume that the length of each side is between 1 and 199 meters. It's from a chapter in my calculus book called "Application of dervatives". Would someone be able to explain how this kind of problem works and how to solve it? Thank you!!
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