Question

What's the formula for circumference of a circle?

Answer 1

If a circle has radius `R`, its circumference equals to `2piR`, where `pi` is an irrational number that, approximately, equals to `3.1415926`

Explanation:

The most interesting part is, obviously, how this formula can be obtained.
I suggest you to watch a lecture on UNIZOR Geometry - Length and Area - Circumference of a Circle that explains in details how this formula can be derived.