Pi Symbol
The Greek letter π (pi) is used in trigonometry as a constant to represent a halfrotation around a circle in radians. The value of π is approximately 3.14 and appears in the geometric formulas for finding the circumference and area of the circle. The value of π can be calculated by dividing any circle's circumference by its diameter.
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Unicode  π 

Usage
The area of a circle is given by π (pi) times the radius of the circle squared.
The circumference of a circle is given by two multiplied by pi and the radius.
The greek letter π (pi) is a naturally occuring number that is defined by any circle's circumference divided by its diameter.
Related Symbols
The Greek letter τ (tau) is used in trigonometry as a constant to represent a full rotation around a circle in radians. The value of τ is approximately 6.28 and can be calculated by dividing any circle's circumference by its radius.