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The Two Circumference Formulas

publish date: 2026/05/17 03:18:33.539735 UTC

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The circumference of a circle can be calculated using either the diameter \(D\) or the radius \(r\). Study the labeled circle below.

If the radius is known, which formula is used to find the circumference?

\(C = \pi D\)

\(C = 2\pi r\)

\(C = \pi r^2\)

\(C = 4r\)

Correct Answer

\(C = 2\pi r\)

Explanation

There are two equivalent circumference formulas: \(C = \pi D\) (when the diameter is known) and \(C = 2\pi r\) (when the radius is known). They are equivalent because \(D = 2r\). The formula \(C = \pi r^2\) is for area, not circumference.

Reference

Mathematics for college students

Quizzes you can take where this question appears

Find the circumference of a circle