http://edugen.wileyplus.com/edugen/courses/crs7165/art/qb/qu/c29/pict_29_93.gif if I wanted to find the area of the outer circular strip, I know I would just take the area of the composite and subtract out the inner radius. (i.e. pi(a^2)-pi(b^2)). However why cant I use 2pi*radius*thickness? this would be 2pi*a(a-b) which is obviously not the same.