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• Perimeter of an equilateral triangle = 3 × length of its side

= 3a units a a
Example 1: Find the perimeter of the following:

(a) Equilateral triangle: length of side = 7.2 cm a

(b) Rectangle: Length = 17 cm, breadth = 9 cm
(c) Square: length of side = 2.4 m

Solution: (a) Perimeter of an equilateral triangle = 3 × length of its side
= 3 × 7.2 cm = 21.6 cm [Q side = 7.2 cm]

(b) Length of rectangle = 17 cm, breadth of rectangle = 9 cm
We have, Perimeter of the rectangle = 2 × (length + breadth) units

Perimeter of the rectangle = 2 × (17 + 9) cm = 2 × 26 cm = 52 cm

(c) Perimeter of the square = 4 × length of its side
= 4 × 2.4 m = 9.6 m [Q side = 2.4 m]

Example 2: The length and breadth of a rectangular field are 128 m and 84 m respectively. What
is the cost of 3 rounds of fencing the wire at the rate of `15 per metre?

Solution: Length of the rectangular field, l = 128 m,
Breadth of the rectangular field, b = 84 m Quick Check

The perimeter of a squared field
We have, perimeter of the rectangular field
of side 65 m is equal to the
= 2 × (length + breadth) perimeter of a rectangular field
whose length is 85 m. Find the
= 2 × (128 + 84) m breadth of the rectangular field.

= 2 × 212 m = 424 m
Now, the required length of fencing wire = 3 × 424 m = 1272 m

Since, the cost of fencing of the field is `15 per metre.

Therefore, the total cost of fencing the field = `(15 × 1272) = `19,080
Area

The region enclosed by the boundary of a closed plane figure is called the area. The unit of area
2
2
2
is given by square unit or unit , i.e., sq. cm (cm ), sq. m (m ), etc.
Area of a rectangle:
Let l be the length and b be the breadth of a rectangle, then,
Area of a rectangle = length × breadth = l × b sq. units

Area of a square:
If a be the length of the side of a square, then

Area of a square = side × side = a sq. units
2

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