# Surface Area of a Frustum

1. Nov 1, 2011

### ElijahRockers

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.