Largest pen against a river

MediumCalculus~4m

You have 200200 metres of fencing to build a rectangular pen along a straight river. The river forms one full side, so no fence is needed there - fencing covers only the other three sides. What is the largest area (in square metres) you can enclose?

Show hints (2)+
  1. The river side is free: 2x+y=2002x + y = 200, so y=200−2xy = 200-2x.
  2. Maximise A=x(200−2x)A = x(200-2x).

Answer

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5000

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