Which one? There are a lot of ways to calculate Pi, although most of them are power series and a pain in the arse to type without LaTeX.
pi = Area/r^2, with r as radius, Area as area enclosed.
r^2 = (x-a)^2+(y-b)^2
pi = Area/r^2, with r as radius, Area as area enclosed.
r^2 = (x-a)^2+(y-b)^2
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