



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 
