Question

# 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.

Solution

## Given:$$l=4$$ cmCircumference of  a circular end =$$18$$ cm$$\Rightarrow 2\pi r_1=18$$$$\Rightarrow \pi \times r_1=\dfrac{18}{2} =9$$ ...............................(1)Circumference of other circular end  $$=6$$ cm$$\Rightarrow 2\pi r_2=6$$$$\Rightarrow \pi r_2=\dfrac{6}{2}=3$$..........................................(2)Adding (1) and (2)Curved surface area $$=\pi (r_1+r_2) l$$$$=(9+3) \times 4$$$$=48$$ cm$$^2$$MathematicsRS AgarwalStandard X

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