The slant height of a frustum of a cone is 4 cm and the perimeters (circumference) of its circular ends are 18 cm and 6 cm. Find the curved surface area of the frustum.

Slant height, l = 4 cm (Given)

\(2 \pi r_1 \ = \ 6 \ \)

\( \Rightarrow \ \pi r_1 \ = \ 3 \)

and \(2 \pi r_2 \ = \ 18 \ \)

\( \Rightarrow \ \pi r_2 \ = \ 9 \)

Curved surface of the frustum \(= \ ( \pi r_1 \ + \ \pi r_2)l \)

\(= \ (3\ + \ 9) \ × \ 4 \ = \ 48 \) cm²