Question:
A solid frustum is of height 8 cm. If the radii of its lower and upper ends are 3 cm and 9 cm respectively, then its slant height is
(a) 15 cm
(b) 12 cm
(c) 10 cm
(d) 17 cm
Solution:
$r_{1}=9 \mathrm{~cm}$
$r_{2}=3 \mathrm{~cm}$
$h=8 \mathrm{~cm}$
Slant height of frustum, $l=\sqrt{h^{2}+\left(r_{1}-r_{2}\right)^{2}}$
$=\sqrt{8^{2}+(9-3)^{2}}$
$=\sqrt{64+36}$
$=\sqrt{100}$
$=10 \mathrm{~cm}$
Hence, the correct answer is choice (c).