A circle has a radius because each radius is the same. A square does not because if it did, not every radius would be the same. Since a square is a regular polygon, it has an apothem, which goes from the center to the middle of a side, so it doesn't need a radius.
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Half the square root of the square radius equals the circle radius.
The same as half the side of the square, as the radius of the circle is half its diameter, and the diameter of the circle is equal to the side of the square.
If a circle has area of 400 square units then its radius is: 11.25 units.
The radius of a circle with an area of 706.50 square inches is: 15 inches.
If the radius of a circle is 80 inches, then the area of the circle is 139.6263 square feet. (rounded)