Pi has been around for a long time. Textual evidence dates it all the way back to 1900 BC. There has been many estimates of Pi and what it could be, but most of them were either inaccurate or not close enough.
But an Ancient Greek mathematician, Archimedes of Syracuse, is the first to calculate an accurate estimation of Pi.
He used his method of finding the area of two polygons. One that's inscribed inside a circle and one where the circle is circumscribed. But of course, thanks to computers later on, Pi was more accurate.
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It was also used in construction of ancient structures. The Great Pyramid of Giza, one of the famous monuments, has a ratio of approximately 2 times Pi.
Today, Pi is used in pop culture as well! Movies, shows, etc. would have certain genres that Pi can be used in. It's used at school and it even has its own holiday, Pi Day.