<P> The number π ( / paɪ / ) is a mathematical constant . Originally defined as the ratio of a circle 's circumference to its diameter , it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics . It is approximately equal to 3.14159 . It has been represented by the Greek letter `` π '' since the mid-18th century , though it is also sometimes spelled out as `` pi '' . </P>
<P> The number π ( / paɪ / ) is a mathematical constant . Originally defined as the ratio of a circle 's circumference to its diameter , it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics . It is approximately equal to 3.14159 . It has been represented by the Greek letter `` π '' since the mid-18th century , though it is also sometimes spelled out as `` pi '' . </P>
when was pi first used in its present form
when was pi first used in its present form
since the mid-18th century
since the mid-18th century