3.9cm. The Radius is always half the diameter of a circle.
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The area of a circle is calculated with the formula πr2, where r is the radius. Therefore, the area of a circle with radius 6cm would be π(6)2 = (36)π, or approximately 113.097 square centimeters.
Radius of a circle= Perimeter of the circle/2*pi Radius is half the diameter Radius is the length of a straight line from the center point of the circle to the edge of the circle.
If it is a 7.1 cm radius circle, then its radius is exactly 7.1 cm!
The diameter of a circle is twice the radius. A circle with a radius of 9 would have a diameter of 18. (9*2=18).
the circumfrence of a circle is pi times it's radius squared.