Definition

"The perimeter of a figure is the total distance around the edge of the figure".
Example:

Perimeter By Using A Formula

The perimeter of some polygons can be calculated directly by using a formula.
Let us calculate the perimeter of a rectangle and square by using a formula.

PERIMETER OF A RECTANGLE

In a rectangle, the perimeter is the sum of all its sides.

Therefore, perimeter        = l + b + l + b
                                 = l + l + b + b
                                 = 2× l + 2 × b
                                 = 2 (l + b)

Examples:

1. Find the perimeter of a rectangle whose length is 5 cm and breadth is 4 cm.
Solution:

Here, length of the rectangle = 5 cm
Breadth of the rectangle = 4 cm
Perimeter of the rectangle = 2 (length + breadth)
= 2 (5 cm + 4 cm)
= 2 (9 cm)
= 18 cm.
Hence, the perimeter of the rectangle is 18 cm.

2. Find the length of the wire required to fence 3 rounds of a rectangular ground of length 20 m and breadth 15 m. Also find the cost of fencing the ground at the rate of ₨ 2 per metre.
Solution: Length of the ground = 20 m
Breadth of the ground = 15 m
Perimeter = 2 (length + Breadth)
= 2 (20 + 15)
= 2(35)
= 70m
Total wire required to fence 3 rounds of ground
= 3 × 70 m = 210 m
Cost of fencing 1m = ₨ 2
Cost of fencing 210 m = 210 × 2
Cost of fencing = ₨ 420

PERIMETER OF A SQUARE

"The sum of all the sides of a square is its perimeter".

Therefore, Perimeter of a square = 4s = 4 × side
Examples:
1. Find the perimeter of a square whose side is 13 cm.
Solution:

Side of a square = 13 cm
Perimeter of a square = 4 × side
= 4 × 13 cm
= 52 cm.

2. Find the side of a square whose perimeter is 36 cm.
Solution: Perimeter of the square = 36 cm
Perimeter = 4 × side
36 = 4 × side
Side = 36 ÷ 4
Side = 9 cm.

PERIMETER OF A TRIANGLE

"The sum of measure of three sides of a triangle is its perimeter".

So, Perimeter of a triangle, P = a + b + c
Example: Find the perimeter of a triangle whose sides are 11 cm, 13 cm, and 8 cm
Solution:

Perimeter of a triangle = sum of its sides
= 11 + 13 + 8
= 32 cm
So, the perimeter of the triangle = 32 cm.