The Greek letter (pi) is used in trigonometry as a constant to represent a halfrotation around a circle in radians. The value of is approximately . The symbol appears in multiple geometric formulas.
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π  Unicode 

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The greek letter π (pi) is a geometric constant approximately equal to 3.1456. The numeric value is equal to the length of any circle's circumference divided by its diameter.
The circumference of a circle is given the constant π (pi) multpilied by two times the radius of the circle.
The traditional formula for the area of a circle is given in terms of the geometric constant π (pi).
Radians are a unit that measure angle using the radius of a circle. One radian is equal to the amount of rotation required to travel the length of one radius along the circumference of the circle.
The capital Greek letter Pi (Π) is used in algebra to represent the product operator. The product operator has three parts: the initial index value, an end value, and the expression being multiplied together.