The circumference (not circumfrence!) is given by the equation
C = 2*pi*r where r is the radius.
So r = C/(2*pi) where pi is the irrational number that is approx 3.14159 or even more approximately, 3.14.
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radius = circumference/(2*pi)
the circumfrence of a circle is pi times it's radius squared.
Remember the eqution and comit to memory C = 2pir or C = pi d Where 'C' is the circumference 'r' is the radius 'd' is the diamter. and 'pi' is the constant of proportion of circles at 3.141592.... usually approximated to 3.14 e.g. If the radius is 5 Then C = 2 * 3.14 * 5 C = 10*3.14 C = 31.4
The radius of a cylinder given only the height could be anything you like.
radius of curvature = 2Focal length