2. You didn't mention if "the radius" belongs to the incircle of the nonagon or the circumscribed circle.
3. The formula you used gives the area of a regular nonagon with an incircle of r = 8.
4. To derive a general formula to calculate the area consider a regular n-gon as n isosceles congruent triangles. You can calculate the base and the height of one triangle using the radius of the circle.