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If you squint hard enough, the capital letter pi (Π) looks like half a circle.
(discuss.tchncs.de)
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It is half of the circumference, multiplied by the radius, of course.
like in this image I found online
That's not half the circumference then is it???
Now you're just being silly.
I can level up with more sources (stanford, wiki).
The circumference is 2pi x radius, which obviously means half the circumference is pi x radius.
Edit: formatting. Asterisk results in italics
https://en.m.wikipedia.org/wiki/Pi