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#### Class nine Mathematics Chapter 13 NCERT Solutions

## Chapter 13: Surface Areas and Volumes

## TEXTBOOK’S EXERCISE – 13.4

**8. A hemispherical bowl is made of steel, 0.25 cm thick. The inner radius of the bowl is 5 cm. Find the outer curved surface area of the bowl. **

**Sol.
**Inner radius of the bowl (r)
= 5 cm

Thickness of the steel = 0.25 cm

∴ Outer radius of the bowl (R)

= (5 + 0.25) cm = 5.25 cm

Outer curved surface
area of the bowl = 2πR^{2}

= 2 × 22/7
× (5.25)^{2} cm^{2} = 173.25 cm^{2}

**9. A right circular cylinder just encloses a sphere of radius r (see figure). Find **

**(i) surface area of the sphere, **

**(ii) curved surface area of the cylinder, (iii) ratio of the areas obtained in (i) and (ii). **

**Sol.
**Here, radius of the sphere = r

Radius of the cylinder = r

And, height of the cylinder = 2r

(i)
Surface area of the sphere = 4πr^{2}

(ii) Curved surface area of the cylinder

= 2πrh = 2π × r × 2r = 4πr^{2}

(iii) Required ratio

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