Circumference and area of circle

Circumference and Area of a Circle

1. circles

2. Parts of a circle Distance around the outside = Circumference (C) Centre Diameter (D) Radius (r) Area (A) The space inside the boundary of a circle =

3. Rules for circumference of a circle Circumference (C) Diameter (D) Radius (r) C = D   C = 2 r   Use the button on your calculator   Circumference = x Diameter   Circumference = 2 x x radius  

4. Examples 12 cm 7mm Find the circumference of these circles (rounding your answer to 2 decimal places): 1. 2. C = D   C =   C = 37.69911184 C = 37.70 cm C = 2 r   C = 2 7   C = 43.98229715 C = 43.98 mm

5. Perimeter of a semi-circle Perimeter = distance around the outside of a shape The perimeter of a semi-circle is made up of a straight line and a curved line . Straight line = diameter Curved line = half of the circumference rule Perimeter = half the circumference + diameter P = + D  

6. Perimeter of a quarter-circle Perimeter = distance around the outside of a shape The perimeter of a quarter-circle is made up of 2 straight lines and a curved line . Straight line = radius Curved line = quarter of the circumference rule Perimeter = quarter of the circumference + 2 x radius P = + 2r  

7. Examples 1. 2. P = + D   P = + D   P = + 9   P = 23.14 m P = + 2r   P = + 2r   P = + 2 x 3   P = 10.71 cm

8. Rule for area of a circle Area (A) Diameter (D) Radius (r) A = r 2   Use the button on your calculator   Area = x radius squared  

9. Examples 12 cm 7mm Find the area of these circles (rounding your answer to 2 decimal places): 1. 2. A = r 2   A = x 7 2   A = 153.93804 A = r 2   A = x 6 2   A = 113.0973355 Area = 153.94 mm 2 Radius = 6cm Area = 113.10 cm 2

10. area of a semi-circle Diameter Radius = half of the diameter rule Area = half the area of a full circle A = ½ r 2   Or you can use A = r 2 ÷ 2  

11. area of a quarter-circle Straight line = radius rule Area = quarter of the full area of a circle A = ¼ r 2   Or you can use A = r 2 ÷ 4  

12. Examples 1. 2. A = ½ r 2   A = ½ x x 4.5 2   A = 31.80862562 Area = 31.81 m 2 A = ¼ r 2   A = ¼ x x 3 2   A = 7.06853471 Area = 7.07 cm 2

13. The end