Chapter 11: Surface Areas and Volumes

Q1Written answer

The curved surface area of a right circular cylinder of height 14 cm is 88 cm². Find the diameter of the base of the cylinder (Assume π =22/7 ).

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Worked solution

Height of cylinder, h = 14cm

Let the diameter of the cylinder be d.

The curved surface area of cylinder = 88 cm²

We know that the formula to find the Curved surface area of the cylinder is 2πrh.

So 2πrh =88 cm² (r is the radius of the base of the cylinder)

2×(22/7)×r×14 = 88 cm²

2r = 2 cm

d =2 cm

Therefore, the diameter of the base of the cylinder is 2 cm.

Q2Written answer

It is required to make a closed cylindrical tank of height 1m and base diameter 140cm from a metal sheet. How many square metres of the sheet are required for the same? Assume π = 22/7

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Worked solution

Let h be the height and r be the radius of a cylindrical tank.

Height of cylindrical tank, h = 1m

Radius = half of diameter = (140/2) cm = 70cm = 0.7m

Area of sheet required = Total surface area of tank = 2πr(r+h) unit square

= [2×(22/7)×0.7(0.7+1)]

= 7.48 square metres

Therefore, 7.48 square metres of the sheet are required.

Q3Written answer

A metal pipe is 77 cm long. The inner diameter of a cross-section is 4 cm, the outer diameter being 4.4cm (see fig. 13.11). Find its

Ncert solutions class 9 chapter 13-3

(i) inner curved surface area

(ii) outer curved surface area

(iii) total surface area

(Assume π=22/7)

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Worked solution

Let r₁ and r₂ inner and outer radii of the cylindrical pipe.

r₁= 4/2 cm = 2 cm

r₂= 4.4/2 cm = 2.2 cm

Height of cylindrical pipe, h = length of cylindrical pipe = 77 cm

(i) Curved surface area of the outer surface of pipe = 2πr₁h

= 2×(22/7)×2×77 cm²

= 968 cm²

(ii) Curved surface area of the outer surface of pipe = 2πr₂h

= 2×(22/7)×2.2×77 cm²

= (22×22×2.2) cm²

= 1064.8 cm²

(iii) Total surface area of pipe = inner curved surface area+ outer curved surface area+ area of both circular ends of pipe

= 2r₁h+2r₂h+2π(r₁²-r₂²)

= 9668+1064.8+2×(22/7)×(2.2²-2²)

= 2031.8+5.28

= 2038.08 cm²

Therefore, the total surface area of the cylindrical pipe is 2038.08 cm².

Q4Written answer

The diameter of a roller is 84 cm, and its length is 120 cm. It takes 500 complete revolutions to

move once over to level a playground. Find the area of the playground in m² (Assume π = 22/7).

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Worked solution

A roller is shaped like a cylinder.

Let h be the height of the roller and r be the radius.

h = Length of roller = 120 cm

Radius of the circular end of roller = r = (84/2) cm = 42 cm

Now, CSA of roller = 2πrh

= 2×(22/7)×42×120

= 31680 cm²

Area of field = 500×CSA of roller

= (500×31680) cm²

= 15840000 cm²

= 1584 m².

Therefore, the area of the playground is 1584 m².

Q5Written answer

A cylindrical pillar is 50 cm in diameter and 3.5 m in height. Find the cost of painting the curved surface of the pillar at the rate of Rs. 12.50 per m².

(Assume π = 22/7)

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Worked solution

Let h be the height of a cylindrical pillar and r be the radius.

Given:

Height cylindrical pillar = h = 3.5 m

Radius of the circular end of pillar = r = diameter/2 = 50/2 = 25cm = 0.25m

CSA of pillar = 2πrh

= 2×(22/7)×0.25×3.5

= 5.5 m²

Cost of painting 1 m² area = Rs. 12.50

Cost of painting 5.5 m² area = Rs (5.5×12.50)

= Rs.68.75

Therefore, the cost of painting the curved surface of the pillar at the rate of Rs. 12.50 per m² is Rs 68.75.

Q6Written answer

Curved surface area of a right circular cylinder is 4.4 m². If the radius of the base of the cylinder is 0.7 m, find its height. (Assume π = 22/7)

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Worked solution

Let h be the height of the circular cylinder and r be the radius.

The radius of the base of the cylinder, r = 0.7m

CSA of cylinder = 2πrh

CSA of cylinder = 4.4m²

Equating both equations, we have

2×(22/7)×0.7×h = 4.4

Or h = 1

Therefore, the height of the cylinder is 1 m.

Q7Written answer

The inner diameter of a circular well is 3.5m. It is 10m deep. Find

(i) its inner curved surface area.

(ii) the cost of plastering this curved surface at the rate of Rs. 40 per m².

(Assume π = 22/7)

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Worked solution

Inner radius of circular well, r = 3.5/2m = 1.75m

Depth of circular well, say h = 10m

(i) Inner curved surface area = 2πrh

= (2×(22/7 )×1.75×10)

= 110 m²

Therefore, the inner curved surface area of the circular well is 110 m².

(ii) Cost of plastering 1 m² area = Rs.40

Cost of plastering 110 m² area = Rs (110×40)

= Rs.4400

Therefore, the cost of plastering the curved surface of the well is Rs. 4,400.

Q8Written answer

In a hot water heating system, there is a cylindrical pipe of length 28 m and diameter 5 cm. Find the total radiating surface in the system. (Assume π = 22/7)

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Worked solution

Height of cylindrical pipe = Length of cylindrical pipe = 28m

Radius of circular end of pipe = diameter/ 2 = 5/2 cm = 2.5cm = 0.025m

Now, CSA of cylindrical pipe = 2πrh, where r = radius and h = height of the cylinder

= 2×(22/7)×0.025×28 m²

= 4.4m²

The area of the radiating surface of the system is 4.4m².

Q9Written answer

Find

(i) the lateral or curved surface area of a closed cylindrical petrol storage tank that is 4.2 m in diameter and 4.5m high.

(ii) How much steel was actually used, if 1/12 of the steel actually used was wasted in making the tank? (Assume π = 22/7)

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Worked solution

Height of cylindrical tank, h = 4.5m

Radius of the circular end , r = (4.2/2)m = 2.1m

(i) The lateral or curved surface area of the cylindrical tank is 2πrh.

= 2×(22/7)×2.1×4.5 m²

= (44×0.3×4.5) m²

= 59.4 m²

Therefore, the CSA of the tank is 59.4 m².

(ii) Total surface area of tank = 2πr(r+h)

= 2×(22/7)×2.1×(2.1+4.5)

= 44×0.3×6.6

= 87.12 m²

Now, Let S m² steel sheet be actually used in making the tank.

S(1 -1/12) = 87.12 m²

This implies, S = 95.04 m²

Therefore, 95.04m²steel was used in actuality while making such a tank.

Q10Written answer

In fig. 13.12, you see the frame of a lampshade. It is to be covered with a decorative cloth.

The frame has a base diameter of 20 cm and a height of 30 cm. A margin of 2.5 cm is to be given for folding it over the top and bottom of the frame. Find how much cloth is required to cover the lampshade. (Assume π = 22/7)

Ncert solutions class 9 chapter 13-4

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Worked solution

Say h = height of the frame of the lampshade, which looks like a cylindrical shape.

r = radius

Total height is h = (2.5+30+2.5) cm = 35cm and

r = (20/2) cm = 10cm

Use the curved surface area formula to find the cloth required for covering the lampshade, which is 2πrh.

= (2×(22/7)×10×35) cm²

= 2200 cm²

Hence, 2200 cm² cloth is required to cover the lampshade.

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Exercise 13.2