Circumference = 3.14 x diameter
Diameter = radius x 2
So......
3.14 x 132 = 414.48
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The radius is 10.5
21 inches
The circumference of a 21-inch circle is: 66 inches.(C = d x Pi).
By a 21-inch circle I presume you mean that the diameter is 21 inches. So the radius is half as long, 10.5 inches. The formula for the circumference of a circle is 2 pi r, where r is the radius. Pi is about 3.14. That is accurate to 2 decimals, or 3 significant digits all together. I use that approximation because the dimension we are given has 2 significant digits. So the circumference is pi times the diameter; we multiply 3.14 by 21 inches and get 65.94. To 2 significant digits, that is 66 inches. If you meant that the radius was 21 inches, then the circumference would be twice as much, or 132 inches.
The formula for the circumference, (C) of a circle is C = 2πr.....where r is the radius. Let R be the radius of the larger circle and r the radius of the smaller circle. Then 66 = 2πR - 2πr = 2π(R - r) Then R - r = 66/2π = 33/π ≅ 10.504 The difference between the two radii is 10.504 (3dp)