Question

what is the origin of pi? where does it come from? How is it used?

Answer 1

Ancient mathematicians noted long ago that the ratio of the circumference to the diameter of a circle was about 3.1416, and that it was an "irrational" number that could not be written exactly as a fraction. Methods were developed to calculate it with arbitrary accuracy by adding up infinite series.

It has many uses in mathematics and physics.

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http://mathforum.org/dr.math/faq/faq.pi.html