<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <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> Being an irrational number, π cannot be expressed exactly as a common fraction (equivalently, its decimal representation never ends and never settles into a permanent repeating pattern). Still, fractions such as 22 / 7 and other rational numbers are commonly used to approximate π . The digits appear to be randomly distributed . In particular, the digit sequence of π is conjectured to satisfy a specific kind of statistical randomness, but to date no proof of this has been discovered . Also, π is a transcendental number; that is, a number that is not the root of any non-zero polynomial having rational coefficients . This transcendence of π implies that it is impossible to solve the ancient challenge of squaring the circle with a compass and straightedge . </P>

When was pi first used in its present form