How do you get the circumference in using Diameter and Radius?

Answer 1

circumference = 2 pi r

so 2 x pi x r

don't need diameter

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Or you could use the diameter only, using this version of the formula

C = pi x d

You'll note that d is 2 x r, so effectively you're just using a rearranged form of the C = 2 x pi x r formula.