Question

The slant height of a frustum of a cone is 4 cm and the circumferences of its circular ends are 18 cm and 6 cm. What is the curved surface area of the frustum?

• A. None of these
• B. $24c{m}^2$
• C. $48c{m}^2$
• D. $96c{m}^2$

Answer 1

The correct option is C $48c{m}^2$

Slant height l = 4 cm,
circumferences = 18 cm and 6 cm
Radii can be calculated as follows:
$Radius=\frac{Perimeter}{2\pi }$
$Or,R=\frac{18}{2\pi }=\frac{9}{\pi }$
$andr=\frac{6}{2\pi }=\frac{3}{\pi }$
Curved surface area of frustum
$=\pi (R+r)l$
$=\pi (\frac{9}{\pi }+\frac{3}{\pi })4$
$=48c{m}^2$