Geometry Calculating Circle Circumference
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Calculating Circle Circumference

 

 Math types sometimes think of a circle

 as a polygon with an infinite number of sides.
 

 

 
 Exactly how is kind of technical,
 so don't worry about it for now.
 
 But what we do need to worry about
 are a few names of measurements on a circle.
 
 A line from the center of a circle to the edge
 is called a radius.
 

 

 
 If you put two of them together to make a straight line 
 going across a circle, you get a diameter.
 

 

 
 The line that is the line of the circle has two names.
 It is called the circumference AND the perimeter.
 

 

 
 There is a special relationship between the circumference
 and the diameter of any circle.
 

 

 
 The circumference is just a bit more 
 than 3 times as long as the diameter.
 
 No matter what size the circle is,
 the circumference is always that same thing.
 Just a bit more than 3 times the diameter.
 
 That three plus a bit number number is very special in math. 
 It is even given a special name. 
 It is called Pi (pronounced Pie)
 We use the Greek letter p to stand for pi.
 
 The value of p is about 3.14159.
 
 Example:
 We have a circle with a diameter of 4.
 What is the circumference of the circle?
 

 

 
 When we want to be really correct we use instead of =.
 The means "about equal to."
 The deal is, pi is an irrational number.
 That means the decimal places go on forever.
 We can never get it exactly right.
 We have to round it somewhere.
 That means any answer we get using a number for p
 will be about right.
 But it will never be exactly right.
 If we have to be exactly right,
 we need to leave the p in the answer.
 
 Example:
 The radius of some circle is 5.
 What is the circumference?
 

 

 The diameter is twice as long as the radius.
 

 

 
 The circumference is equal to the diameter times p.
 

 

 
 Or we can say ...
 

 

 

   copyright 2005 Bruce Kirkpatrick

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