There are many approximations for Pi
If you have a circle, you can measure two things: the distance around the perimeter of the circle (circumference) and the distance across the widest part of the circle (diameter). No matter how big your circle, the ratio of circumference to diameter is the value of Pi. Pi is an irrational number—you can't write it down as a non-infinite decimal. This means you need an approximate value for Pi.
The simplest approximation for Pi is just 3. Yes, we all know that's incorrect, but it can at least get you started if you want to do something with circles. In the past, many math books listed Pi as 22/7. Again, this is just an approximation but it is better than the value of 3 (actually 22/7 is closer to Pi than just writing 3.14).
The early history of mathematics covers many approximations of the value of Pi. The most common method would be to construct a many-sided polygon and use this to calculate the perimeter and diameter as an estimate for Pi. Other cultures found ways to write Pi as an infinite series—but without a computer, this can be sort of difficult to calculate out very far.
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