Question 1151004
Area of the semi-circle = (pi * r^2)/2
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(pi * r^2)/2 = 400
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r^2 = 800/pi
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r = 15.9577 m
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Circumference of the semi-circle = (pi * diameter)/2 = pi * 15.9577 = 50.1326 m
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Area of a rectangle = length * width 
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Note all squares are rectangles but all rectangles are not squares
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A square will yield the minimum perimeter, in this case we only need to consider three sides since the beach will be the other side   :-)
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Area of the square = s^2 = 400, therefore the side is 20 m
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The perimeter required is 60 m, therefore the circumference(50.13226 m) of semi-circle is < the perimeter required for the square(minus one side for the beach) 60 m
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