The slant height of the frustum of a cone is 4 cm and the circumferences of its circular ends are 18 cm and 6 cm. Find curved surface area of the frustum.
It is given that slant height of frustum of a cone is 4 cm. Circumferences of its ends are
18 cm and 6 cm.
We have to find the curved surface area of the frustum.
Let l be the slant height
Let be the radii of two circular ends of the cone
Circumference of one end
Circumference of other end
$2 \pi r=6$
$2 \pi R=18$
$\pi R+\pi r=9+3 \pi R+\pi r=9+3$
$\pi(R+r)=12 \quad \pi(R+r)=12$
Curved surface area of frustum $=\pi(r+R) l$
$=12 \times 4($ since $\pi(R+r)=12)$