The height of a cone is $21\ cm$. Find the area of the base if the slant height is $28\ cm$.

The height of a cone is $21\ cm$ and the slant height is $28\ cm$.

To do:

We have to find the area of the base.

Solution:

Height of the cone $(h) = 21\ cm$

Slant height of the cone $(l) = 28\ cm$

Therefore,

$l^2 = r^2 + h^2$

$r^2 = l^2 - h^2$

$= (28)^2 - (21 )^2$

$=784 - 441$

$= 343$….......(i)

Area of the base $= \pi r^2$

$= \frac{22}{7} \times 343$              [From (i)]

$= 22 \times 49$

$= 1078\ cm^2$