The diameters of the ends of a frustum of a cone are 32 cm
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).