Viète's formula (https://en.wikipedia.org/wiki/Vi%C3%A8te%27s_formula), an infinite product of nested roots evaluating to \(2/\pi\), "the first formula of European mathematics to represent an infinite process". Now another Good Article on Wikipedia.
(More GA's to come soon, I hope. I had been trying to keep my nominations and reviews in balance but there were too few nominations to review on topics of interest, so I started nominating more often, and they're having a drive to clean out old nominations.)
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