9 inches.
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If the diameter of a circle is 18 inches then its radius is 18/2 = 9 inches
Radius squared is half of the diameter multiplied by itself. Example: diameter is 6 inches, radius is 3 inches (half of diameter), radius squared is 9 inches (radius multiplied by itself 3 x 3 = 9).
Radius = Diameter/2. So in this case 4.5 inches.
It is the name given to the straight line from the centre of a circle to its circumference when that distance is 36 inches.
Since your circle has a diameter of 9 feet (the definition of a "9 by 9" circle), the radius is half of that distance. In this case, your radius is 4.5 feet or 54 inches.