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NCERT solution class 9 chapter 13 Surface Areas and Volumes exercise 13.9 mathematics

EXERCISE 13.9


Question 1:

A wooden bookshelf has external dimensions as follows: Height = 110 cm, Depth = 25 cm, Breadth = 85 cm (see the given figure). The thickness of the plank is 5 cm everywhere. The external faces are to be polished and the inner faces are to be painted. If the rate of polishing is 20 paise per cm2 and the rate of painting is 10 paise per cm2, find the total expenses required for polishing and painting the surface of the bookshelf.

Answer:

External height (l) of book self = 85 cm

External breadth (b) of book self = 25 cm

External height (h) of book self = 110 cm

External surface area of shelf while leaving out the front face of the shelf

= lh + 2 (lb bh)

= [85 × 110 + 2 (85 × 25 + 25 × 110)] cm2

= (9350 + 9750) cm2

= 19100 cm2

Area of front face = [85 × 110 − 75 × 100 + 2 (75 × 5)] cm2

= 1850 + 750 cm2

= 2600 cm2

Area to be polished = (19100 + 2600) cm= 21700 cm2

Cost of polishing 1 cm2 area = Rs 0.20

Cost of polishing 21700 cm2 area Rs (21700 × 0.20) = Rs 4340

It can be observed that length (l), breadth (b), and height (h) of each row of the book shelf is 75 cm, 20 cm, and 30 cm respectively.

Area to be painted in 1 row = 2 (hb + lh

= [2 (75 + 30) × 20 + 75 × 30] cm2

= (4200 + 2250) cm2

= 6450 cm2

Area to be painted in 3 rows = (3 × 6450) cm= 19350 cm2

Cost of painting 1 cm2 area = Rs 0.10

Cost of painting 19350 cm2 area = Rs (19350 × 0.1)

= Rs 1935

Total expense required for polishing and painting = Rs (4340 + 1935)

= Rs 6275

Therefore, it will cost Rs 6275 for polishing and painting the surface of the bookshelf.

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Question 2:

The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in the given figure. Eight such spheres are used for this purpose, and are to be painted silver. Each support is a cylinder of radius 1.5 cm and height 7 cm and is to be painted black. Find the cost of paint required if silver paint costs 25 paise per cm2 and black paint costs 5 paise per cm2.

Answer:

Radius (r) of wooden sphere = 

Surface area of wooden sphere = 4πr2

Radius (r1) of the circular end of cylindrical support = 1.5 cm

Height (h) of cylindrical support = 7 cm

CSA of cylindrical support = 2πrh

Area of the circular end of cylindrical support = πr2 

= 7.07 cm2

Area to be painted silver = [8 × (1386 − 7.07)] cm2

= (8 × 1378.93) cm2 = 11031.44 cm2

Cost for painting with silver colour = Rs (11031.44 × 0.25) = Rs 2757.86

Area to be painted black = (8 × 66) cm= 528 cm2

Cost for painting with black colour = Rs (528 × 0.05) = Rs 26.40

Total cost in painting = Rs (2757.86 + 26.40)

= Rs 2784.26

Therefore, it will cost Rs 2784.26 in painting in such a way.

Question 3:

The diameter of a sphere is decreased by 25%. By what per cent does its curved surface area decrease?

Answer:

Let the diameter of the sphere be d.

Radius (r1) of sphere 

CSA (S1) of sphere =

CSA (S2) of sphere when radius is decreased =

Decrease in surface area of sphere = S1 − S2


 

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