Question

why do we sometimes use 2pi instead of pi when doing volumes revolution?

Answer 1

in converting radians to degree, the conversion factor is 180/pi. Hence, in a complete revolution which is 360, there is 2pi. I hope that answers your question.

360 (pi/180)
=2pi

Answer 2

not really ....when doing maths .specificaly calculus in volumes of revolution on the shell's method we use 2pi but on the diskwashers method we use pi