The Total solution for NCERT class 6-12
The slant height ofa frustum of a cone is 4 cm and the perimeters (circumference) of its circularends are 18 cm and 6 cm. Find the surface area of the frustum.
Given,
Slant height (l) = 4 cm
Circumference of upper circular end of thefrustum = 18 cm
∴ 2πr1 = 18
Or, r1 = 9/π
Similarly, circumference of lower end of thefrustum = 6 cm
∴ 2πr2 = 6
Or, r2 = 6/π
Now, CSA of frustum = π(r1+r2)× l
= π(9/π+6/π) × 4
= 12×4 = 48 cm2