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# 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