02 Mensuration
02 Mensuration
2 MENSURATION
INTRODUCTION :
With each plane figure, we think of their regions and their boundaries. For expression of two plane
figures, some measures are needed. For example Perimeter, Area etc.
Ø Perimeter : The concept of perimeter is widely used in our daily life . For example,
Þ in fencing a field.
Þ in preparing a track to conduct sports
Þ in building a compound wall etc.
“PERIMETER” is the distance covered along the boundary forming a closed figure when you go
round the figure once.
Ø Perimeter of a rectangle = 2 × (l + b) . Where l, b are the length and breadth
1
Þ Length of a rectangle = × ( Perimeter) – breadth
2
1
Þ Breadth of rectangle = × (Perimeter) – length
2
Ø Perimeter of a square = 4 × side of square
1
Þ side of a square = × perimeter
4
Ø Perimeter of a scalene triangle is the sum of all the three sides of it.
P = a + b + c. Where a, b, c are the sides.
Perimeter of an equilateral triangle with side “a” is 3a. Perimeter of any polygon is equal to the sum
of all its sides.
The figures having all sides of equal length and all angles of equal measure are known as “regular
figures”. Example : square, equilateral triangle
Regular figures perimeter = number of sides × length of one side
SOLVED EXAMPLES
1. Find the perimeter of each of the following figures:
a) perimeter = the sum of the lengths of the sides = 4 cm + 1 cm + 3 cm + 2 cm + 3 cm + 4 cm
+ 1 cm + 3 cm + 2 cm + 3 cm + 4 cm + 1 cm + 3 cm+ 2 cm + 3 cm + 4 cm + 1 cm + 3 cm +
2 cm + 3 cm = 52 cm
4 cm
1 cm
3 cm
3 cm
4 cm
3 cm
2 cm
3 cm
2 cm
3 cm
2 cm
2 cm
4 cm
3 cm
3 cm
3 cm
1 cm
4 cm 1 cm
4
cm
cm
4
4 cm
4 cm
4 cm
8. Sweety runs around a square park of side 75 m. Bulbul runs around a rectangular park with
length 60m and breadth 45 m. Who covers less distance?
Sol: The distance each girl covers in one round is the same as the perimeter of respective field.
\ The distance that Sweety covers in one round
= 4 × side [square field]
= 4 × 75 m [since side = 75 m]
= 300 m.
The distance that Bulbul covers in one round
= 2 × (length + breadth) [rectangular field]
= 2 × (60m + 45 m) [since l = 60m; b = 45 m]
= 2 × 105 m
= 210 m.
Hence,
Sweety covers 300m.
Bulbul covers 210 m.
So, this shows that Bulbul covers less distance than Sweety.
9. Find the perimeter of regular octagon with each side measuring 9cm.
Sol: Regular octagon has all 8 sides equal.
so, number of sides = 8
length of each side = 9 cm.
Perimeter = 8 × length of each side
= 8 × 9 cm
= 72 cm
10. The length of a rectangle is twice of its breadth. If its perimeter is 48cm find the dimensions of
the rectangle?
Sol: Let the breadth of a rectangle be “ x” cm
Then the length = twice of the breadth
= 2 x cm.
Perimeter of a rectangle = 2 × (length + breadth)
= 2 × (2x + x) cm
= 2 × 3x cm
= 6x cm.
But according to problem, the perimeter is 48 cm.
So, 6x cm = 48 cm.
8
48
x= cm
61
x = 8 cm.
i.e, Breadth = 8 cm
Length = 2x.
= 2 × 8 cm
= 16 cm.
\ The dimensions of a rectangle are 16cm, 8 cm.
EXERCISE
1. Find the perimeter of the given shape.
F D
m
6c
3c
4
cm
m
C
5 cm
E
4c
m
A 8 cm B
AREA
w The amount of surface enclosed by a closed figure is called its “Area”.
w The area of a closed figure can be found by using a graph paper (or) a squared paper.
w While measuring the area on squared paper the following conventions to be adopted.
i) The area of one full square is to be taken as 1 square unit.
1
ii) If the area occupied exactly half a square, it is to be taken as square unit.
2
iii) If the area occupied more than half of a square, it is to be taken 1 square unit.
iv) If the area occupied less than half of a square, we can ignore the portion of that area.
w The units of perimeter is same as the units of the length of the object.
w The units of area is square centimetres if the dimensions of the figure is given centimetres. The units
of area is square metres if the dimensions of the figure is given in metres.
w Areas of smaller figures are measured in sq. cm (or) sq. m
w Areas of larger figures are measured in sq. km (or) hectares (or) ares
w The area of a rectangle = length × breadth
The area of a square = side × side
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SOLVED EXAMPLES
1. Find the area of the rectangles with the given measurements :
a) 50 cm and 16 cm b) 25 cm and 20 cm
Sol: a) Length of a rectangle = 50 cm
Breadth of a rectangle = 16 cm
Area of a rectangle = length × breadth
= 50 cm × 16 cm
= 800 sq.cm
b) Length of a rectangle = 25 cm
Breadth of a rectangle = 20 cm
Area of a rectangle = length × breadth
= 25 × 20
= 500 sq. cm
60
120
⇒ x= 1
2
∴ x = 60 m
Hence, breadth (b) = 60m
length (l) = 20 + x
= 20 + 60 m
= 80 m
Now l = 80 m, b = 60 m
Area of rectangle = l × b
= 80 m × 60 m
= 4800 sq.m
6. What is the cost of tiling a rectangular piece of land 250 m long and 150m wide at the rate of
< 9 per hundred sq. m
Sol: Length of rectangular piece = 250 m
Breadth of rectangular piece = 150 m
Area of rectangular piece = l × b
= 250 m × 150 m
= 37, 500 sq.m
Cost of tiling 100 sq.m = 9
9
∴ cost of tiling 1 sq.m =
100
9
cost of tiling 37,500 sq.m = × 37,500
100
= 3375
7. Five square flower beds each of sides 1m are dug on a piece of land 5m long and 4m wide.
What is the area of the remaining part of the land?
Sol: Length of land = 5m
Breadth of land = 4m
Area of land = l × b = 5 × 4 1m 1m
= 20 sq. m 4m
Side of a square flower bed = 1m 1m 1m 1m
Area of a square flower bed = 1m× 1m = 1 sq.m
Area of 5 square flower beds = 5 × 1 sq. m = 5 sq.m 5m
Hence, area of remaining part of land
= 20 sq. m – 5 sq. m
= 15 sq.m
10. How many tiles with dimensions 8 cm and 15 cm will be needed to fit a region whose length
and breadth are respectively: 200 cm and 120 cm.
Sol: Length of region = 200cm
breadth of region = 120 cm
area of region = l × b
= 200 cm × 120 cm
= 24000 sq.cm
Length of tile = 15 cm
breadth of tile = 8 cm
area of tile = l × b = 15 cm × 8 cm = 120 sq.cm
Number of tiles required
Area of region
=
Area of one tile
2400 sq.cm
= = 200
120 sq.cm
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11. Split the following shapes into rectangles and find the area of each in square centimetres.
7
3 1
5 7 7
1 1 7 7 2
2 2 2 4
7 7
a) b) c) 4
4 4 7 7 4 3
1 7 7 3
7
Sol: a) Area of the shape = [(5 × 1) + (4 × 1)] sq. cm 5
1 1 1
= 5 + 4 sq. cm 2 2
4 4
= 9 sqcm 1
b) Area of the shape = [ (7 × 7) + (7 × 7) + (7 × 7) + (7 × 7) + (7 × 7) ] sq. cm
7
= 49 + 49 + 49 + 49 + 49 sq. cm
7 7
7 7 7
= 5 × 49 sq. cm
7 7 7 7
7
= 245 sq.cm 7 7
c) Let the figure may be divided into rectangles marked as A, B, C and D. 7 7
7
Area of rectangle (A) = (3 × 3) sq . cm
= 9 sq. cm (7-5) = 2
Area of rectangle (B) = (1 × 2) sq .cm 3 1 D
= 2 sq. cm 2 4
Area of rectangle (C) = (3 × 3) sq. cm 2 C 3
= 9 sq. cm 1 B 11 2 2
3 4
Area of rectangle (D) = (4 × 2) sq. cm 4 3 A 3
= 8 sq. cm 3
∴ The total area of the figure,
= ( 9 + 2 + 9 + 8) sq. cm
= 28 sq. cm
PRACTICE EXERCISE
1. Find the area of the rectangle whose length and breadth are 2m and 70 cm.
2. The area of a rectangular garden is 260 sq.m and its width is 13 m find its length.
3. Find the area of a square plot whose side is 19 cm.
4. Which of the following has a larger area and by how much? (a) A rectangle of length 21 cm and
width 11 cm, or (b) A square of side 18 cm
5. A tile measures 32 cm by 25 cm. How many tiles will be required to cover a floor of size 4m by 3 m.
2m
7. The perimeter of the figure is 2 m 2m [ ]
2m
a) 8 m b) 16 m c) 4 m d) None of these
4m
8. 3m 3 m The perimeter of the figure is [ ]
4m
a) 12 m b) 14 m c) 24 m d) 7 m
9. Area of a rectangle = [ ]
a) length × breadth b) length + breadth
c) 2 × (length + breadth) d) 2 × (length × breadth)
10. Area of a square = [ ]
a) side × side b) 4 × length of a side c) 2 × length of a side d) 6 × length of a side
11. If a page is 25 cm long and 20 cm wide, then the perimeter of this page is [ ]
a) 90 cm b) 45 cm c) 500 cm d) 5 cm
4 cm
m
3c
3 cm
a) 20 cm b) 10 cm c) 24 cm d) 15 cm
13. The cost of fencing a rectangular park of length 10 m and breadth 5m at the rate of 10 per metre
3
5. The length of the rectangle is 80 m and the breadth is of its length. Find the area of rectangle.
4
6. Find the length of a rectangle whose breadth is 48 cm and the area is 1728 sq.cm.
7. Find the perimeter of equilateral triangle with side of length 5 cm.
8. Find the perimeter of an isosceles triangle with equal sides 4 cm each and third side 6 cm.
9. A piece of string is 120 cm long. What will be the length of each side if the string is bent in the form
of a regular hexagon?
10. A square piece of ground is 75 m long. Find the cost of erecting a fence round it at <4 per metre.
KEY
I. Multiple Choice Questions :
1) c 2) a 3) b 4) d 5) d 6) d 7) a 8) b
9) a 10) a 11) a 12) a 13) a 14) c 15) b 16) a
17) c 18) d 19) a 20) a 21) b 22) c 23) b 24) d
25) a
KEY
1) b 2) b 3) c 4) c 5) a 6) a 7) b 8) b
9) c 10) a 11) b 12) b 13) d 14) a 15) c