A solid frustum is of height 8 cm.
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).

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