21/(2*pi) = a radius of about 3.342253805 units
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A circle with a radius of 21 inches has a circumference of 131.95 inches.
The radius of any circle is the straight line distance from the centre of the circle to any point on the circumference of the circle. The radius being 21 cm makes no difference to the definition of the radius; it just means that the straight line distance from the centre of that circle is 21cm away from any point on the circumference of that circle. The radius of a circle with a radius of 21 cm is 21 cm.
If a circle circumference is 30 the radius is: 4.77
The circumference of a circle with a 2pi radius is: 39.58
The radius of a circle with a circumference of 59 is: 9.39