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.

Answers (1)

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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