# Circle

## Important Formulae

Diameter of a circle is usually denoted by ‘D’.
Radius of a circle is denoted by ‘r’, thus, diameter = 2r
Circumference of circle = 2 π r, where r is the radius of circle
Area of circle = 2 π r2

Question 1: Find the circumference of the circles with the following radius: (a) 14 cm (b) 28 mm (c) 21 cm

Solution: (a) Given, radius, r = 14 cm

We know that circumference = 2 π r

= 2 xx (22)/(7)xx 14 = 2 xx 22 xx 2 = 88 cm

(b) Given, radius, r = 28 mm,

We know that circumference = 2 π r

= 2 xx (22)/(7) xx 28 = 2 xx 22 xx 4 = 176 mm

(c) Given, radius, r = 21 cm

We know that circumference = 2 π r

= 2 xx (22)/(7) xx 21 = 2 xx 22 xx 3 = 132 cm

Question 2: Find the area of the following circles, given that: (a) Radius = 14 cm, (b) Diameter = 49 mm, (c) Radius = 5 cm

Solution: (a) Given, radius, r = 14 cm

We know that area of a circle = π r^2

= (22)/(7) xx 14^2

= (22)/(7) xx 14 xx 14

= 22 xx 2 xx 14 = 616 sq cm

(b) Given, diameter, D = 49 mm; or, radius = (49)/(2) mm

Area = π r^2

= (22)/(7) xx (49)/(2) xx (49)/(2)

= 11 xx 7 xx (49)/(2)

= 11 xx 3.5 xx 49 = 1886.5 sq mm

(c) Given, radius, r = 5 cm

Area = π r^2

= (22)/(7) xx 5^2

= 3.14 xx 25 = 78.5 sq cm

Question 3: If the circumference of a circular sheet is 154 m, find its radius. Also find the area of the sheet.

Solution: Given, circumference = 154 m, radius = ?, Area = ?

Circumference of a circle = 2 π r

Therefore, 154 = 2 π r

Or, π r = (154)/(2) = 77

Or, r = (77)/(π) = 77 xx (7)/(22) = 24.5 m

Area = π r^2

= (22)/(7) xx 24.5 xx 24.5

= 22 xx 3.5 xx 24.5 = 1886.5 sq m

Question 3: If the circumference of a circular sheet is 154 m, find its radius. Also find the area of the sheet.

Answer: Given, circumference = 154 m, radius = ?, Area = ?

Circumference of a circle = 2 π r

Therefore, 154 = 2 π r

Or, π r = (154)/(2) = 77

Or, r = (77)/(π) = 77 xx (7)/(22) = 24.5 m

Area = π r^2

= (22)/(7) xx 24.5 xx 24.5

= 22 xx 3.5 xx 24.5 = 1886.5 sq m

Question 4: A gardener wants to fence a circular garden of diameter 21m. Find the length of the rope he needs to purchase, if he makes 2 rounds of fence. Also find the cost of the rope, if it costs Rs 4 per meter.

Solution: Given, Diameter = 21m, cost of the rope = Rs 4 per meter, length of the rope required to fence the garden two round=?, Cost of the rope = ?

The length of rope required = 2 xx circumference.

Circumference = 2 π r = π D

= (22)/(7) xx 21 = 66 m

Therefore, length of rope required = circumference xx 2 = 66 m xx 2 = 132 m

Cost of rope = Rate xx length

= 4 xx 132 = Rs. 528

Question 5: From a circular sheet of radius 4 cm, a circle of radius 3 cm is removed. Find the area of the remaining sheet. (Take π = 3.14)

Solution: Radius of circular sheet = 4cm,

Area = π r^2

= 3.14 xx 4^2 = 3.14 xx 16= 50.24 cm2

Radius of circular sheet which is removed = 3 cm

Area = π r^2

= 3.14 xx 3^2 = 3.14 xx 9 = 28.26 cm2

Now, Area of remaining sheet = Area of whole sheet — Area of removed sheet

= 50.24 cm2 — 28.26 cm2 = 21.98 cm2

Question 6: Question:6. Saima wants to put a lace on the edge of a circular table cover of diameter 1.5m. Find the length of the lace required and also find its cost if one meter of the lace costs Rs 15 (Take π = 3.14)

Solution: Diameter of a circular table = 1.5 m, Cost of one metre of lace = Rs 15

Circumference of circle table = π D

= 3.14 xx 1.5 = 4.71 m

Cost = Rate xx Length

= Rs. 15 xx 4.71 = Rs. 70.65

Copyright © excellup 2014