Perimeter

Forum by Delta publications

key notes :

Definition of Perimeter

The perimeter of a closed figure is the total distance around the figure.

In simple words, it is the sum of the lengths of all sides.

Perimeter of Basic Shapes

Square

  • All sides are equal.
  • Formula:

Perimeter=4×side

Rectangle

  • Opposite sides are equal.
  • Formula:

Perimeter=2×(Length+Breadth)

Triangle

  • Sum of the lengths of all three sides.
  • Formula:

Perimeter=a+b+c

Parallelogram

  • Opposite sides are equal.
  • Formula:

Perimeter=2×(Base+Side)

Rhombus

  • All sides are equal.
  • Formula:

Perimeter=4×side

Circle

  • Perimeter of a circle is called the Circumference.
  • Formula:

Circumference=2πr or πd

(where r=radius,  d=diameter)

Units of Perimeter

Always measured in units of length.

Example: cm, m, km, inches, feet, etc.

Important Points
  • The longer the boundary, the larger the perimeter.
  • Perimeter helps us to find fencing length, wire around shapes, or distance walked around a figure.
Solved Examples

Example 1:
Find the perimeter of a rectangle with length 12 cm and breadth 8 cm

P=2×(12+8)=2×20=40 cm

Example 2:
A square field has a side of 25 m. Find its perimeter.

P=4×25=100 m

Example 3:
Find the circumference of a circle of radius 7 cm

C=2πr=2×22/7​×7=44cm

Practice Questions
  1. Find the perimeter of a triangle with sides 8 cm,6 cm,10 cm
  2. A rectangular park has length 40 m and breadth 25 m. Find its perimeter.
  3. Find the perimeter of a square having side 15 m
  4. Find the circumference of a circle of diameter14cm.

Learn with an example

➡️ What is the perimeter of this parallelogram?

_____  centimetres

In a parallelogram, opposite sides have the same length. Look at the parallelogram and find the side lengths. add:

8 + 10 + 8 + 10 = 36

Now find the units. The lengths are measured in centimetres, so the perimeter is also measured in centimetres.

The perimeter is 36 centimetres.

➡️ What is the perimeter?

______  kilometres

Look at the shape and find the length of each side. Add the lengths:

20 + 16 + 12 = 48

Now find the units. The lengths are measured in kilometres, so the perimeter is also measured in kilometres.

The perimeter is 48 kilometres.

➡️ Look at this parallelogram. Find the value of a.

Perimeter=18 centimetres

a= _______ centimetres

In a parallelogram, opposite sides have the same length. Add the lengths of the sides:

a + 6 + a + 6 = 18

2a + 12 = 18

Subtract 12 from each side. Then, divide by 2.

2a=6

Now look for the units. The units are centimetres. So, a = 3 centimetres.

Let’s practice!🖊️