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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=4cm
Circumference of a circular end =18cm⇒2πr1=18⇒π×r1=18/2=9.. (1)
Circumference of other circular end=6cm⇒2πr2=6⇒πr2=6/2=3. (2)
Adding (1) and (2)
Curved surface area=π(r1+r2)l=(9+3)×4=48cm2