For a circle with radius r

 

Area of circle will be

  Area of circle = πœ‹ r 2

 

Here,

  πœ‹  can be taken as

  πœ‹ = 3.14 or πœ‹ = 22/7

 

Let’s take some examples

38.jpg

 

Find area of circle of radius 2 cm by counting squares , and also by formula

39.jpg

 

Covered Area

Number

Area

1

Fully - filled squares

4

1 × 4 = 4

2

Half - filled squares

0

1/2 × 0 = 0

3

More than Half - filled squares

8

1 × 8 = 8

4

Less than Half - filled squares

4

0 × 4 = 0

40.jpg

Area = 4 + 0 + 8 + 0

 = 12 cm 2

By formula of area of circle

Area = πœ‹r 2

 = 3.14 × (2) 2

 = 3.14 × 4

 = 314/100 × 4

         = 1256/100

         = 12.56 cm 2

41.jpg

So, finding area by formula is more accurate

 

Find area of a circle with radius of 3.5 cm

42.jpg

Radius = r = 3.5 m

 

Area = πœ‹r 2

= πœ‹ (3.5) 2

= 22/7 × (35/10) 2

= 22/7 × (7/2) 2

= 22/7 × 7/2 × 7/2

= 11/7 × 7/1 × 7/2

= 11/1 × 1 × 7/2

= 77/2

= 38.5 cm 2

∴ Area is 38.5 cm 2

 

Find area of a circle whose radius of 7 cm

43.jpg

Radius = r = 7 cm

  Area = πœ‹ r 2

  = 22/7 × (7) 2

  = 22/7 × 7 × 7

  = 22 × 7

  = 154 cm 2

 

44.jpg

 ∴ Area is 154 cm 2

 

Find area of a circle with diameter 21 cm

45.jpg

Diameter = 21 cm

Radius = r = Diameter/2

    = 21/2 cm

Now,

  Area = πœ‹ r 2

          = 22/7 × (21/2) 2

          = 22/7 × 21/2 × 21/2

   = 11/7 × 21/1 × 21/2

   = 11/1 × 3 × 21/2

   = (33 × 21)/2

   = 693/2

           = 346.5 cm 2

46.jpg

∴ Area is 346.5 cm 2

 

Find area of a circle with diameter 42 cm

47.jpg

Diameter = 42 cm

Radius = r = Diameter/2

= 42/2 cm

= 21 cm

 

Now,

  Area = πœ‹ r 2

= 22/7 × (21) 2

= 22/7 × 21 × 21

= 22/1 × 3 × 21

= 66 × 21

= 1386 cm 2

 

48.jpg

∴ Area is 1386 cm 2

 

Find area of a circle with radius 5 cm

49.jpg

Radius = r

 = 5 cm

 

Now,

  Area = πœ‹ r 2

  = 3.14 × (5) 2

  = 3.14 × 25

  = 314/100 × 25

  = 314/20 × 5

  = 314/4 × 1

  = 157/2

  = 78.5 cm 2

∴ Area is 78.5 cm 2

 

Find the area of a circle with diameter 30 cm

50.jpg

Diameter = 30 cm

Radius = r = Diameter/2

= 30/2 cm

= 15 cm

 

Now,

  Area = πœ‹ r 2

   = 3.14 × (15) 2

  = 3.14 × 225

  = 314/100 × 225

  = 314/20 × 45

  = 157/10 × 45

  = 7065/10

  = 706.5 cm 2

∴ Area = 706.5 cm 2

 

 

 

 

 

 

  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.