54 inches
Because:
sin30=x/9 sin30 is the degrees that makes the apothems angle of depression and 9 is the radius.
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Circumference of a circle: 2*pi*radius or diameter*pi
just over 56½ inches (18 pi)
If the radius is 18 feet - then the diameter is 36 feet. The circumference measurement is irrelevant !
(pie)(diameter)=(circumference) (pie)(9)(2)=18(pie)=56.549
All depends what you mean by 18 inches, is that the circumference, the diameter or the radius