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## Homework Statement

My professor put on the board today, that the area of a ring (used in discussion of moments of inertia) was = the circumference of the ring *dr = 2*pi*r*dr.

This may sound trivial, but I cannot seem to work out in my head how this related to the formula I know for the area of a ring = pi*(r^2_outer-r^2_inner). I see the units are ok in either, but the factor of 2 is confusing me.

Thanks