Question : The slant height of a cone is 20 cm. If the area of its base is 616 cm2, then what is the curved surface area of this cone? (use $\pi=\frac{22}{7}$)
Option 1: 960 cm2
Option 2: 400 cm2
Option 3: 1760 cm2
Option 4: 880 cm2
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Correct Answer: 880 cm 2
Solution :
Given: The slant height of a cone is 20 cm.
The area of its base is 616 cm
2
.
The curved surface area of the cone is $\pi rl$ where $r$ and $l$ are its radius and slant height respectively.
According to the question,
$\pi r^2=616$
⇒ $\frac{22}{7}\times r^2=616$
⇒ $r^2=\frac{7}{22}\times616$
⇒ $r^2=196$
⇒ $r=14$ cm
The curved surface area of the cone
$=\frac{22}{7}\times14\times20=22\times 2\times 20=880$ cm
2
Hence, the correct answer is 880 cm
2
.
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