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The Greek letter π (lower case) or Π (upper case), equivalent in sound to the English p.
π is commonly used to represent the ratio of the circumference of a circle to its diameter.
That is, π = [C/d]
It"s value is approximately 3.142 or [22/7]. The exact value cannot be represented as a fraction. The decimal expansion goes on forever without recurring.
π is therefore a useful shorthand for a number that is otherwise hard to write down.
π occurs in many formulae, notably: C = πD , C = 2πr and
A = πr² , where r is the radius of the circle and A is its area.
To a few dozen decimal places, the value of π is
3,1415926535897932384626433832795028841971693993751058209749445923078164062862 08998628034825342117068.

Π denotes a product of values in the same way as Σ is used to indicate a sum of values.

Π_r=1^k r = k! = 1 ×2 ×... ×(k−1) ×k.