# 2. 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.

We are given the perimeter of upper and lower ends thus we can find r1 and r2.

$2\pi r_1\ =\ 18$

$r_1\ =\ \frac{9}{\pi}\ cm$

And,

$2\pi r_2\ =\ 6$

$r_2\ =\ \frac{3}{\pi}\ cm$

Thus curved surface area of the frustum is given by  :      $=\ \pi \left ( r_1\ +\ r_2 \right )l$

$=\ \pi \left ( \frac{9}{\pi}\ +\ \frac{3}{\pi} \right )4$

$=\ 48\ cm^2$

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