Pi is the ratio of a circle's circumference to its radius squared.
The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159.
In this sense one speaks of the diameter rather than a diameter -LRB- which refers to the line itself -RRB- , because all diameters of a circle or sphere have the same length , this being twice the radius r.	circle	circle	sphere	sphere	radius	radius
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