Like we use πD to get the circumefrence of the circle but why we use π please explain

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Make a circle with a rope on the floor, the distance from the center to the border; it takes 3,1415…. Times to make it a circle.

Pi is a mathematical property of plane geometry. It’s related to the shape we call a circle. It’s driven by math identities.

It is a magic number that is always the same and is what you multiply by how “wide” a circle is to get how “long” the outside of it is.

As for why it is that specific value, it is just how the universe works, there isn’t a real reason as for why it could not be a different value. Perhaps in a different universe where the laws of mathematics and physics are different, it might somehow be different. Who knows?

As for why it uses the greek value pi, we often use greek characters in mathematics (like capital pi for ‘all the things multiplied together’, capital sigma for ‘all the things added together’, etc).

A long time ago, people realized that no matter what size circle you have, the ratio between the circumference and the diameter is always the same. This ratio is now defined as pi. It ends up being useful in many places, especially in mathematics and physics when dealing with stuff involving circles, arcs, and rotation. It is an irrational number, and many people are fascinated with its never-ending stream of digits when representing it in decimal form.

Pi is, essentially be definition, the ratio of a circle’s circumference to its diameter. So it’s really pi = c/D by definition, so once you know the value of pi then c=piD. You can use this definition to calculate pi by taking polygons with more and more sides (getting closer and closer to a circle). It’s not a very efficient way to do it but it works.

Pi is much more mathematically fundamental than that, it shows up in all kinds of places you might not expect it, but comes down to the basic definition.