The Greek scholar Archimedes first measured pi by determining the l...
http://www.chacha.com/question/howwaspifirstme...

First, a comment: "measurement" is probably the wrong term here. It implies some sort of physical process of measuring something which isn't the case here at all. Although
http://www.quora.com/Howdomathematiciansarrive...

PI is ratio of circumference to radius of a circle. Since to arrive at ratio both dimensions are measured in same units of length. Therfore it has no measurement unit of PI. P=22/
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