18mm or 1.8 cm
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Diameter = 2 x radius = 9 meters
The radius is 9 inches because area of circle is pi*9 squared = 81pi square inches and the diameter is twice the radius which is 2*9 = 18 inches
the diameter would be 9 you would do 4.5+4.5 and that would = 9
I'm wondering what the original, or book, question actually asks, but in this case, a it is a circle larger in radius, diameter and circumference by the ratio 4:3; and in area by the ratio 16:9.
Doubling the radius doubles the circumference.