Question:
The radius of the base of a cone is 5 cm and its height is 12 cm. Its curved surface area is
(a) 60π cm2
(b) 65π cm2
(c) 30π cm2
(d) None of these
Solution:
(b) 65π cm2
Given: r = 5 cm, h = 12 cm
Slant height of the cone, $l=\sqrt{r^{2}+h^{2}}$
$=\sqrt{(5)^{2}+(12)^{2}}$
$=\sqrt{25+144}$
$=\sqrt{169}$
$=13 \mathrm{~cm}$
Hence, the curved surface area of the cone $=\pi r l$
$=(\pi \times 5 \times 13) \mathrm{cm}^{2}$
$=65 \pi \mathrm{cm}^{2}$
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