Question

The surface area of a frustum cone is 2,200 cm2. The larger and smaller radius of the cone is 20 and 10 cm. find its slant height. (Use $\pi$ = 22/7).

• A. 7.666 cm
• B. 6.666 cm
• C. 8.666 cm
• D. 9.666 cm

Answer 1

The correct option is B 6.666 cm

Surface area of a frustum of cone $=\pi$(r + R)s + $\pi$r${}^2$ + $\pi$R${}^2$

R = 20cm, r = 10cm, SA = 2,200 cm2

2,200 $=$ 22/7 $\times$ (10+ 20)s +$\pi$10${}^2$ + $\pi$20${}^2$

2,200 $=$ 22/7 $\times$ [30s +100 + 400]

2,200 $=$ 22/7 $\times$ [30s +500]

2,200 $\times$ 7/22$=$ 30s + 500

700 500 $=$ 30s

200/30 $=$ s

S $=$ 6.666 cm