Question

The surface area of the frustum cone is $297c{m}^2$ whose top circle radius is $2$ cm and bottom circle radius is $5$ cm. Find its slant height. (Use $\pi =3$)

• A. $10$ m
• B. $8$ cm
• C. $9$ cm
• D. $10$ cm

Answer 1

Answer: (D)

$10$ cm

Surface area of frustum cone $=\pi (r+R)s+\pi r^2+\pi R^2$
$297=\pi (2+5)s+\pi 2^2+\pi 5^2$
$297=(3\times 7)s+3\ast 4+3\times 25$
$297=21s+12+75$
$297=21s+87$
$297-87=21s$
$210÷21=s$

Slant height, $s=10$ cm