Class 8 Maths NCERT Solutions Chapter 11 – Mensuration

Class 8 Maths NCERT Solutions Chapter 11: Mensuration

Ex – 11.1

Question 1.
A square and a rectangular field with measurements as given in the figure have the same perimeter. Which field has a larger area?


Solution.
Area of the square field = a x a
= 60 m x 60 m = 3,600 \({ m }^{ 2 }\)
Perimeter of the square field = 4a
= 4 x 60 m = 240 m
∴ Perimeter of rectangular field = 240 m
⇒ 2(l + b) = 240
⇒ 2(80 + b) = 240
where b m is the breadth of the rectangular field
⇒ 80 + b = \(\frac { 240 }{ 2 } \) ⇒ 80 + bx = 120
⇒ b = 120 – 80 = 40
∴ Breadth = 40 m
∴ Area of rectangular field
= l x b = 80 m x 40 m = 3,200 \({ m }^{ 2 }\)
So, the square field (a) has a larger area

Question 2.
Mrs. Kaushik has a square plot ‘ with the measurement as shown in the figure. She wants to construct a house in the middle of the plot. A garden is developed around the house. Find the total cost of developing a: garden around the house at the rate of ₹ 55 per \({ m }^{ 2 }\).
Solution.
Area of the square plot = a x a
= 25 x 25 \({ m }^{ 2 }\) = 625 \({ m }^{ 2 }\)
Area of the house = a x b
= 20 x 15 \({ m }^{ 2 }\) = 300 \({ m }^{ 2 }\)
∴ Area of the garden
= Area of the square plot – Area of the house
= 625 \({ m }^{ 2 }\) – 300 \({ m }^{ 2 }\)
= 325 \({ m }^{ 2 }\)
∵ The cost of developing the garden per square metre = ₹ 55.
∴ Total cost of developing the garden
= ₹ 325 x 55
= ₹ 17,875.

Question 3.
The shape of a garden is rectangular in the middle and semicircular at the ends as shown in the diagram. Find the area and the perimeter of this garden.

Solution.

Question 4.
A flooring tile has the shape of a parallelogram whose base is 24 cm and the cor-responding height is 10 cm. How many such tiles are required to cover a floor of area 1080 m2? (If required you can split the tiles in whatever way you want to fill up the corners).
Solution.
Area of a flooring tile = bh
= 24 x 10 \({ cm }^{ 2 }\)
= 240 \({ cm }^{ 2 }\)
Area of the floor
= 1080 \({ m }^{ 2 }\)
= 1080 x 100 x 100 \({ cm }^{ 2 }\)
∵ m2 = 100 x 100 \({ cm }^{ 2 }\)

∴ Number of tiles required to cover the floor
=\(\frac { Area\quad of\quad the\quad floor\quad }{ Area\quad of\quad a\quad flooring\quad tile } \)
= \(\frac { 1080\times 100\times 100\quad }{ 240 } \)
= 45000.

Question 5.
An ant is moving around a few food pieces of different shapes scattered on the floor. For which food-piece would the ant have to take a longer round? Remember, the circumference of a circle can be obtained by using the expression c = 2πr, where r is the radius of the circle.


Solution.

Ex – 11.2

Question 1.
The shape of the top surface of a table is a trapezium. Find its area, if its parallel sides are 1 m and 1.2 man the d perpendicular distance between them is 0.8 m.
Solution.
Area of the top surface of the table

= \frac { 1 }{ 2 } h(a+b)
= \frac { 1 }{ 2 } \times 0.8\times (1.2+1)
= 0.88{ m }^{ 2 }

Question 2.
The area of a trapezium is 34 cm2 and the length of one of the parallel sides is 10 cm and its height is 4 cm. Find the length of the another parallel side.
Solution.
Area of trapezium
= \frac { 1 }{ 2 } h(a+b)

⇒ 34=\frac { 1 }{ 2 } \times 4(10+b)
⇒ 34=2\times (10+b)
⇒ 10+b=\frac { 34 }{ 2 }
⇒ 10 + b=17
⇒ b = 17 – 10
⇒ b = 7 cm
Hence, the length of another parallel side is 7 cm.

Question 3.
Length of the fence of a trapezium shaped field ABCD is 120 m. If BC = 48 m, CD = 17 m and AD = 40 m, find the area of this field. Side AB is perpendicular to the parallel sides AD and BC

Solution.
Fence of the trapezium shaped field ABCD = 120 m
⇒ AB + BC + CD + DA = 120
⇒ AB + 48 + 17 + 40 = 120
⇒ AB + 105 = 120
⇒ AB = 120 – 105
⇒ AB = 15 m
∴ Area of the field
= \frac { (BC+AD)\times AB }{ 2 }
= \frac { (48+40)\times 16 }{ 2 } = 660 { m }^{ 2 }

Question 4.
The diagonal of a quadrilateral shaped field is 24 m and the perpendiculars dropped on it from the remaining opposite vertices are 8 m and 13 m. Find the area of the field.

Solution.
Area of the field
= \frac { 1 }{ 2 } d({ h }_{ 1 }+{ h }_{ 2 })
= \frac { 24\times (8+13) }{ 2 } = \frac { 24\times 21 }{ 2 }
= 12 x 21 = 252{ m }^{ 2 }

Question 5.
The diagonals of a rhombus are 7.5 cm and 12 cm. Find its area.
Solution.
Area of the rhombus

= \frac { 1 }{ 2 } \times { d }_{ 1 }\times { d }_{ 2 }
= \frac { 1 }{ 2 } \times 7.5\times 12
= 45 { m }^{ 2 }

Question 6.
Find the area of a rhombus whose side is 5 cm and whose altitude is 4.8 cm. If one of its diagonals is 8 cm long, find the length of the other diagonal.
Solution.
Area of the rhombus
= base (b) x altitude (h) = 5
= 5 x 4.8 = 24 { cm }^{ 2 }

Question 7.
The floor of a building consists of 3,000 tiles which are rhombus shaped and each of its diagonals are 45 cm and 30 cm in length. Find the total cost of polishing the floor, if the cost per { m }^{ 2 } is ₹ 4.
Solution.
Area of a tile

Question 8.
Mohan wants to buy a trapezium shaped field. Its side along the river is parallel to and twice the side along the road. If the area of this field is 10,500 { m }^{ 2 } and the perpendicular distance between the two parallel sides is 100 m, find the length of the side along the river.
Solution.
Let the length of the side along the road be x m. Then, the length of the side along the river is 2x m.
Area of the field = 10,500 square metres

Question 9.
Top surface of a raised platform is in the shape of a regular octagon as shown in the figure. Find the area of the octagonal surface.
Solution.
Area of the octagonal surface

Question 10.
There is a pentagonal shaped park as shown in the figure. For finding its area Jyoti and Kavita divided it in two different ways.

Find the area of this park using both ways. Can you suggest some other way of finding its area ?
Solution.

Diagram of the adjacent picture frame has outer dimensions = 24 cm x 28 cm and inner dimensions 16 cm x 20 cm. Find the area of each section of the frame, if the width of each section the same.

Solution.