Question

The surface area of a frustum cone is 3,300 mm${}^2$. The larger and smaller radius of the cone is 5 and 2 mm. find its slant height. (Use $\pi$ = 22/7)

• A. 135.85 mm
• B. 145.85 mm
• C. 155.85 mm
• D. 165.85 mm

Answer 1

The correct option is B 145.85 mm

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

R = 5mm, r = 2mm, SA = 3,300 mm2

3,300 $=$ 22/7 $\times$ (2 +5)s +$\pi$2${}^2$ + $\pi$5${}^2$

3,300 $=$22/7 $\times$ [7s +4 + 25]

3,300 $=$22/7 $\times$ [7s + 29]

3,300 $\times$ 7/22$=$7s + 29

1,050 29 $=$ 15s

1,021/7 $=$ s

S = 145.85 mm