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It we measure the circumference and diameter of various circles; we will find that the ration of circumference and their corresponding diameters, is always constant. This constant ratio is denoted by Greek letter, . The value of  is  or  .
 
Or 
 Or  ( , being radius)
Example:
The sum of two radii of two circles is  cm and the difference of their circumference is  cm. Find the two circumference.
Solution:
Let  be the radius of one circle.
The radius of the other circle  cm
Now, circumference of circle with radius  cm  cm --- (1)
and, circumference of circle with radius  cm  --- (2)
The difference of two circumference  cm --- (3)
From equations (1), (2) and (3) we get
 
 
 
  cm
Thus, circumference of the circle with radius  cm  cm and circumference of second circle, 
 cm
Example:
What will be the circumference of a circle inscribed an equilateral triangle of side  cm.
Solution:
Let  be the centre of the inscribed circle of 
Since, radius of inscribed circle in a triangle 
Now Area of  
 Semi perimeter of  
 Radius of inscribed circle  cm
 Circumference of the circle 
 cm
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