If you want to ask questions about the "following", then I suggest that you make sure that there is something that is following.
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Radius = Diameter/2 Diameter = 2* Radius
If the radius is 42 meters, then you already know the radius.
divide diameter by 2
Area of any circle is: pi*radius*radius or pi*r^2
Radius is half of the diameter. So radius2 means rxr.