Question : The curved surface area of a right circular cone is $156 \pi$ and the radius of its base is 12 cm. What is the volume of the cone, in cm3?
Option 1: $192 \pi$
Option 2: $210 \pi$
Option 3: $240 \pi$
Option 4: $180\pi$
Correct Answer: $240 \pi$
Solution : Given: The curved surface area of a right circular cone is $156 \pi$ and the radius of its base is 12 cm. Use the formulas, The curved surface area of the cone = $\pi rl$, The volume of the cone = $\frac{1}{3}\pi r^2h$, The slant height of the cone = $l=\sqrt{r^2+h^2}$, where $r$, $h$, and $l$ are the radius, height and slant height respectively. According to the question, $\pi\times 12\times l=156\pi$ ⇒ $l=13$ cm Also, $l^2=r^2+h^2$ ⇒ ${13}^2={12}^2+h^2$ ⇒ $169=144+h^2$ ⇒ $h^2=169–144$ ⇒ $h^2=25$ ⇒ $h=5$ cm The volume of the cone $=\frac{1}{3}\pi\times {12}^2\times 5$ $=12\times 4\times 5\pi =240\pi$ cm 3 Hence, the correct answer is $240 \pi$.
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