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The height of a cone is $21\ cm$. Find the area of the base if the slant height is $28\ cm$.
Given:
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)]
$= 22 \times 49$
$= 1078\ cm^2$
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