What is Pi?

Date: 03/02/97 at 22:51:41
From: Anna
Subject: Pi's

What exactly is pi? Is it a number, an equation, or what?

Anna

Date: 03/02/97 at 23:58:23
From: Doctor Mike
Subject: Re: Pi's

Dear Anna,
  
Pi is a number. It is approximately equal to 3.1416 , but that is not 
its exact value. 

We are interested in this number because of an astounding fact about
all circles. Take an exact circle, any circle.  The distance around it 
is called the "circumference" and the length of a line from one side 
to the other through the center is called the "diameter".  Sometimes 
that line itself is called a diameter line.  Now divide:
   
             circumference 
            ---------------  =  ?
               diameter   
  
What number do you get?  For big circles or smaller circles or medium-
sized circles the answer is ALWAYS the same.  That number is pi.  

I cannot write down here for you exactly what pi is, because I would 
have to write down an infinite number of digits.  I can tell you that 
a better approximate value is : 
   
             3.1415926535898
  
but that still is not exact.  The real pi just keeps going on and on.  
Fortunately, most of the times we use it we only need one of the 
shorter approximate values for it.  

Two of the best known ways to use pi are in formulas for finding the 
circumference "C" of a circle and the area "A" of a circle, if you 
know what the radius "R" of the circle is.  They are : 
    
       C = 2*pi*R
  
       A = pi*R*R
  
where "*" stands for multiplication.  Since R*R is sometimes called "R 
squared", often this equation for area is said "pi R squared".  
  
In summary, pi is a number, but it is used in equations. 

-Doctor Mike,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/