C = 2 x r x pi
Since your circumference is known you can put that into the circumference equation.
18pi = 2 x r x pi
Divide 2 and pi from both sides and you get this
r = 9
your radius of your circle is 9
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The circumference of a circle is 2*pi*radius or diameter*pi
Circumference of a circle: 2*pi*radius or diameter*pi
circumference = 18*pi feet
Circumference of the circle: 2*pi*9 = 18*pi cm
If a circle has a circumference of 18pi, the radius is 9.