You can measure it. Otherwise, you certainly have to know SOMETHING about the circle to calculate the radius, usually the diameter, the circumference, or the area. If you don't know anything at all about a circle, you can't find out the radius, either.
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The radius is exactly 1/2 of the diameter.
The radius is half of the diameter so you divide the diameter in half. (divide by 2) If your trying to find the diameter if you have the radius you double it (multiply by 2)
Area of Circle = pi (radius)2 So , using this formula we can find the radius and also the diameter. Diameter = 2(radius)
True. The radius is half the diameter.
Diameter = 2r If r=0.5, then diameter = 2 x 0.5 = 1