Question:
The diameters of the ends of a frustum of a cone are 32 cm and 20 cm. If its slant height is 10 cm, then its lateral surface area is
(a) 321 π cm2
(b) 300 π cm2
(c) 260 π cm2
(d) 250 π cm2
Solution:
$r_{1}=\frac{32}{2}$
$=16 \mathrm{~cm}$
$r_{2}=\frac{20}{2}$
$=10 \mathrm{~cm}$
Slant height = 10 cm
Total lateral surface area
$=\pi\left(r_{1}+r_{2}\right) l$
$=\pi(16+10) 10$
$=260 \pi \mathrm{cm}^{2}$
Hence, the correct answer is choice (c).