Ok, so I have problems understanding where they came up with the formula S=2∏rL, where r = (1/2)(r1+r2). They say the formula is verified in exercise 62. I did the exercise and showed that S=∏rL. Correct me if I'm wrong, but isn't this contradictory? I can see how they're using the average radius of the frustum, but I'm not sure what allows them to do that. I guess I just need to see a geometric proof before I really get it. Any ideas? Thanks in advance.