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Date: 04/12/97 at 08:33:01
From: Chris Riley 
Subject: Circumference

My class was trying to figure out the answer to this question.  What 
is the circumference of a circle with a radius of 5.5?

Date: 04/12/97 at 15:48:25
From: Doctor Mason
Subject: Re: Circumference

Hi, Chris,

Most of the time we use a formula to calcuate the circumference of a 
circle once either the radius or diameter is known.  To find the 
circumference, you only need to multiply the diameter by a constant 
known as pi.  

Pi is approximately equal to the decimal 3.14 or the fraction 22/7.  
Neither one of these values is exactly equal to pi, since pi is a 
special kind of number (an irrational number), but even though 3.14  
and 22/7 aren't exactly equal to pi, they are close enough to do most 

In the formulas below 

C = circumference
d = diameter
r = radius and 
* means multiply:

   C = pi * d          or  since 2 radius equal a diameter,
   C = pi * 2 * r      or often written   
   C = 2*pi*r

If you know that the radius = 5.5, then according to the last formula:
   C = 2 * 3.14 * 5.5  =  34.54.

Glad to help!  For more information about pi, see our Dr. Math FAQ at   

-Doctor Mason,  The Math Forum
 Check out our web site!   
Associated Topics:
Elementary Circles
Elementary Geometry

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