Students can download Maths Chapter 7 Mensuration Ex 7.2 Questions and Answers, Notes, Samacheer Kalvi 9th Maths Guide Pdf helps you to revise the complete Tamilnadu State Board New Syllabus, helps students complete homework assignments and to score high marks in board exams.

## Tamilnadu Samacheer Kalvi 9th Maths Solutions Chapter 7 Mensuration Ex 7.2

Question 1.

Find the Total Surface Area and the Lateral Surface Area of a cuboid whose dimensions are length = 20 cm, breadth = 15 cm, height = 8 cm.

Solution:

Here l = 20 cm, b = 15 cm, h = 8 cm

L.S.A. of the cuboid = 2(1 + b)h sq.m

= 2(20 + 15) × 8

= 2 × 35 × 8

= 560 sq.m

Total surface area of the cuboid = 2(lb + bh + lh) sq.units

= 2(20 × 15 + 15 × 8 + 8 × 20) sq. cm

= 2(300 + 120 + 160) sq. cm

= 2 × 580 sq. cm

= 1160 sq. cm

Question 2.

The dimensions of a cuboidal box are 6 m x 400 cm x 1.5 m. Find the cost of painting its entire outer surface at the rate of Rs 22 per m².

Solution:

Length of the cuboid box (l) = 6 m

Breadth of the cuboid box (b) = 400 cm = 4m

Height of the cuboid box (h) = 1.5 m

T.S.A. of the cuboid = 2(lb + bh + lh) sq.units

= 2(6 × 4 + 4 × 1.5 + 1.5 × 6) sq.units

= 2(24 + 6 + 9)

= 2 × 39 sq.m

= 78 sq.m

Cost of painting for one sq.m = Rs 22

Total cost of painting = Rs 78 × 22

= Rs 1716

Question 3.

The dimensions of a hall is 10 m × 9 m × 8 m. Find the cost of white washing the walls and ceiling at the rate of Rs 8.50 per m².

Solution:

Length of the hall (l) = 10 m

Breath of the hall (b) = 9 m

Height of the hall (h) = 8 m

Area to be white wash = L.S.A. + Ceiling of the building

= 2(l + b)h + lb sq.units

= 2(10 + 9)8 + 10 × 9 sq.m

= 2 × 19 × 8 + 10 × 9 sq. m

= (304 + 90) sq.m

= 394 sq.m

Cost of white washing one sq.m = Rs 8.50

Cost of white washing for 394 sq.m = Rs 394 × 8.50

= Rs 3349

Total cost of white washing = Rs 3349

Question 4.

Find the TSA and LSA of the cube whose side is

(i) 8 m

(ii) 21 cm

(iii) 7.5 cm

Solution:

(i) 8m

Side of a cube (a) = 8m

T.S.A. of the cube = 6a² sq.units

= 6 × 8 × 8 sq. m

= 384 sq.m

L.S.A. of the cube = 4a² sq.units

= 4 × 8 × 8 sq.m

= 256 sq.m

(ii) 21 cm

Solution:

Side of a cube (a) = 21 cm

T.S.A. of the cube = 6a² sq. units

= 6 × 21 × 21 cm²

= 2646 cm²

L.S.A. of the cube = 4a² sq.units

= 4 × 21 × 21 sq.cm

= 4 × 441 cm²

= 1764 cm²

(iii) 7.5 cm

Solution:

Side of a cube (a) = 7.5 cm

T.S.A. of the cube = 6a² sq.units

= 6 × 7.5 × 7.5 cm²

= 337.5 cm²

L.S.A. of the cube = 4a² sq.units

= 4 × 7.5 × 7.5 sq.cm

= 225 cm²

Question 5.

If the total surface area of a cube is 2400 cm² then, find its lateral surface area.

Solution:

T.S.A. of the cube = 2400 cm²

6a² = 2400

a² = \(\frac{2400}{6}\)

= 400 cm²

L.S.A. of the cube = 4a² sq.units

= 4 × 400 cm²

= 1600 cm²

(OR)

T.S.A. of the cube = 2400 cm²

6a² = 2400

a² = \(\frac{2400}{6}\)

= 400

a = \(\sqrt{400}\)

= 20 cm

Side of a cube (a) = 20 cm

L.S.A. of the cube = 4a² sq.units

= 4 × 20 × 20 cm²

= 1600 cm²

Question 6.

A cubical container of side 6.5 m is to be painted on the entire outer surface. Find the area to be painted and the total cost of painting it at the rate of Rs 24 per m².

Solution:

Side of a cube (a) = 6.5 m

Total surface area of the cube = 6a² sq.units

= 6 × 6.5 × 6.5 sq.m

= 253.50 sq.m

Cost of painting for 1 sq.m = Rs 24

Cost of painting for 253.5 sq.m = 253.5 × 24

= Rs 6084

∴ Area to be painted = 253.50 m²

Total cost of painting = Rs 6084

Question 7.

Three identical cubes of side 4 cm are joined end to end. Find the total surface area and lateral surface area of the new resulting cuboid.

Solution:

Joint the three identical cubes we get a new cuboid

Length of the cuboid (l) = (4 + 4 + 4) cm

l = 12 cm

Breadth of the cuboid (b) = 4 cm

Height of the cuboid (h) = 4 cm

Total surface area of the new cuboid = 2(lb + bh + lh) sq.units

= 2(12 × 4 + 4 × 4 + 4 × 12)

= 2(48 + 16 + 48) cm

= 2(112) cm²

= 224 cm²

Lateral surface area of the new cuboid = 2(l + b)h sq.units

= 2(12 + 4)4 cm²

= 2 × 16 × 4 cm²

= 128 cm²

∴ T.S.A of the new cuboid = 224 cm²

L.S.A of the new cuboid = 128 cm²