The radius is a line from the center of the circel to any point on the outside of the circle.
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A circle has no vertices around it
"Diameter" is the distance all the way across the circle, through the center."Radius" is 1/2 of the diameter ... the distance from the center to the curvy part of the circle."Circumference" is the distance all the way aroundthe circle.
Given the diameter of the circle, multiply it by π (approx 3.14159).
Find the area of both circles (A = πr2) and subtract the area of the larger circle from that of the smaller circle inside it.
You need to measure it with something flexible, like a string.If you know for sure it is a circle, it would be easier to measure the diameter, then multiply by pi.