The internal and external radius of a hemisphere are r and R respectively.Find the total surface area of the solid. The internal and external radius of a hemisphere are r and R respectively.Find the total surface area of the solid. Now this all seems to be very easy as CSA of this hemisphere=2*pi*r^2+2*pi*R^2 what i cannot find out is the reaming area.

Hint: The "flat" part lies in a plane, and consists of a disk of radius $R$ with a disk of radius $r$ removed from its center. What's the area of the resulting annular region?