Question

The radius of the base of a cone is $4\text{cm}$ and the height is $3\text{cm}$. Find its Curved Surface area. $(\pi =\frac{22}{7})$

• A. $20{\text{cm}}^2$
• B. $20\pi {\text{cm}}^2$
• C. $30\pi {\text{cm}}^2$
• D. None of these

Answer 1

Answer: (B)

$20\pi {\text{cm}}^2$

Curved Surface area of cone = $\pi rl$,

where $r$ is the radius of the cone,

$l$ is the slant height of the cone

By Pythagoras theorem, slant height $l$ can be calculated as follows:

$l=\sqrt{r^2+h^2}$

$=\sqrt{4^2+3^2}$

$=\sqrt{25}$

$=5$

Curved Surface Area $=\pi rl=\frac{22}{7}\times 5\times 4{\text{cm}}^2$

$=62.85{\text{cm}}^2$

$=20\pi {\text{cm}}^2$