What is the solution to Dido's problem?

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SUMMARY

Dido's problem addresses the geometric shape that maximizes the area enclosed by a fixed perimeter with one side as a straight line. The conclusion is that a semicircle does not yield the maximum area; instead, a full circle would be optimal if no straight side constraint existed. However, under the conditions of Dido's problem, the optimal shape is a semicircle, as it utilizes the straight line as its diameter. The area calculations confirm that a semicircle with a diameter of 100 m results in an area of approximately 594.2 m², while a full circle cannot be used due to the straight side requirement.

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pilpel
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Given a fixed perimeter, what is the geometric shape (non-self-intersecting closed curve) that would maximize the enclosed area? MathWorld (I'm not allowed to link) and a few other sites insist that the answer is a semi-circle, but simple math with perimeter=100 m would show that a circle would have area 2500/\pi\approx795.8 m^{2} whereas a semicircle wold have area \approx594.2 m^{2}.

What am I missing?
 
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Dido's problem demands that one side of the enclosure is a straight line of fixed length. A circle would not qualifiy, since it has no straight sides.

If you allow all closed curves, then the circle would be optimal.
 

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