Answer:
 52.500 by 52.500 inches
Step-by-step explanation:
The rectangle with maximum area will be a square. Its side length will be 1/4 the perimeter, so is 210/4 = 52.5 inches.
The figure is a 52.500 inch square. The interval of optimization is closed.
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Side lengths are restricted to the interval 0 to 105 inches.
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Any n-sided polygon with a given perimeter will have its maximum area when the polygon is regular. A regular 4-gon is a square.