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I know that the ratio of the circumference to the diameter is Pi - what about the ratio of the circumference to the radius? Does it have any practical purpose when we have Pi? Is it called something (other than 2 Pi)?

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What does "does it mean anything" mean? Does 17 mean anything? Please clarify your question. – Gerry Myerson Aug 22 '11 at 2:25
"I know that the ratio of the circumference to the diameter is Pi" is incorrect, should also specify in Euclidean geometry to that sentence, try finding the ratio of circumference to the diameter on a sphere just for fun. – Arjang Aug 22 '11 at 8:15
@Gerry Good point. I've removed that part. – Odinulf Aug 22 '11 at 13:42
is this D= D-r with chasles? is this only 4 of all of the Unité de mesure. theory of all – user52413 Jan 23 '15 at 16:43
and there is something about 3D max and DDr is a triangle and you can only see a segment with r=D/2 – user52413 Jan 23 '15 at 16:49
up vote 18 down vote accepted

The ratio of the circumference to the radius is $2\pi$, which some people call "One turn". I think you would enjoy to read this article: "$\pi$ is wrong!" by Bob Palais. Other people call $2\pi$ by the name of Tau. See this page:

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Thanks - those websites were very interesting. – Odinulf Aug 22 '11 at 14:03

Note that, by definition, $$\text{diameter}=2\cdot\text{radius},$$ so that $$\pi=\frac{\text{circumference}}{\text{diameter}}=\frac{\text{circumference}}{2\cdot\text{radius}}=\frac{1}{2}\cdot\left(\frac{\text{circumference}}{\text{radius}}\right),$$ or in other words, $$\frac{\text{circumference}}{\text{radius}}=2\pi.$$

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That $\pi$ and $2 \pi$ have a very simple relationship to each other sharply limits the extent to which one can be more useful or more fundamental than the other.

However, there are probably more formulas that are simpler when expressed using $2\pi$ instead of $\pi$, than the other way around. For example, there is often an algebraic expression involving something proportional to $(2\pi)^n$ and if expressed using powers of $\pi$ this would introduce factors of $2^n$.

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