Let us take a circle

24.jpg

Distance around the circle is called circumference of circle.

So, Perimeter of circle is actually circumference of circle.

 

For a circle with radius r,

  Circumference = 2πœ‹r

Here,

     r is the radius,

and

     πœ‹ = pi

It’s value is

        πœ‹ = 3.14159265359……

 

We write it as

        πœ‹ = 3.14 or πœ‹ = 22/7

 

Let’s take some examples

 

Find circumference of circle with radius 3.5 cm

25.jpg

 

Radius = r

            = 3.5 cm

Circumference = 2πœ‹r

 = 2 × 22/7 × 3.5

 = 2 × 22/7 × 35/10

 = 2 × 22/1 × 5/10

 = 22 × 5/5

 = 22 × 1 = 22 cm

 

Find circumference of circle with radius 7.0 cm

26.jpg

Radius = r

            = 7 cm

 

Circumference = 2πœ‹r

 = 2 × 22/7 × 7

 = 2 × 22/1 × 1

 = 44 cm

 

Find circumference of circle with diameter 21 cm

27.jpg

Diameter = 21 cm

Radius = r = Diameter/2

    = 21/2

 

Circumference = 2πœ‹r

 = 2 × 22/7 × 21/2

 = 22/7 × 21/1

 = 22 × 3

 = 66 cm

 

Find circumference of circle with diameter 42 cm

28.jpg

Diameter = 42 cm

   Radius = r = Diameter/2

= 42/2

= 21 cm

 

Circumference = 2πœ‹r

 = 2 × 22/7 × 21

 = 2 × 22/1 × 3

 = 44 × 3

 = 132 cm

 

Find circumference of circle with radius 5 cm

29.jpg

Radius = r

            = 5 cm

 

Circumference = 2πœ‹r

 = 2 × 3.14 × 5

 = 3.14 × 5 × 2

 = 3.14 × 10

 = 31.4 cm

 

Find circumference of circle with diameter 30 cm

30.jpg

Diameter = 30 cm

Radius = r = Diameter/2

  = 30/2

  = 15 cm

Circumference = 2πœ‹r

 = 2 × 3.14 × 15

 = 3.14 × 2 × 15

 = 314/100 × 30

 = 314/10 × 3

 = 942/10

 = 94.2 cm

  1. Chapter 11 Class 7 Perimeter and Area
  2. Concept wise

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.