The raduis.It's pi, r squared. the r stands for radius!
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Well, you find the area of a circle by multiplying (pi times radius2) or (pi times radius times radius). The radius is half of the diameter. The formula for the circumference is (pi times diameter). Multiply the radius times 2 to get the diameter. Multiply the diameter times pi.
If that's the diameter, multiply the diameter by pi (which is about 3.1416). If it's the radius, multiply it by (2 x pi).
Circumference = Pi times diameter. So double the radius to find the diameter. Multiply by Pi (3.14159)
Multiply the radius by 2 to get the diameter. Then multiply the diameter by pi to get the circumference. Example: If radius = 2 inches, diameter = 4 inches circumference = 4 x pi = 4 (3.1416) or about 12.57 inches
You have to multiply the diameter by pi (3.14) If you have the area, that means you multiplied pi by radius squared. Now you just have to multiply the radius by two, then multiply that number by pi.