The diameter of the circle is used for the square construction, but the radius of the circle is used for the regular hexagon construction wich is A.
The diameter of the circle is used for the square as l=2r therefore corresponds to the diameter, while for a hexagon r=r being the radius of the circle.
How to calcule the diameter?
If you know the radius of the circle, multiply by 2 to get the diameter. The radius is the distance from the center of the circle to the edge. For example, if the radius of a circle is 4 cm, then the diameter is 4 cm x 2 = 8 cm. If you know the circumference of the circle, divide by π to get the diameter.
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