The Circumference of a Circle

1

2

3

13 d7C

dC diameter(d)

Circumference (C)

How many times does the diameter fit around the circumference? Choose your number.

1 1½ 2 2½

3 3½

4

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679. .... . . . . . . . . . .π =

d

17

radius

In terms of the radius:

C =? C =2πr

It’s the same for all circles!

The Circumference of a Circle

The Circumference of a Circle

Find the circumference of the following circles.

C = πd

C = 2πr

8 cm

1

9.5 cm

2

C = πd

C= π x 8

C = 25.1 cm (1 dp)

C = πd

C= π x 9.5

C = 29.8 cm (1 dp)

C = 2πr

C = 2 x π x 3

C = 18.8 mm (1 dp)

C = 2πr

C = 2 x π x 2.1

C = 13.2 m (1 dp)

The Circumference of a Circle

Find the circumference of the following circles.

C = πd

C = 2πr

3 mm

4

2.1 m

3

23 cm

7.5 cm

Find the circumference of the tyre and steering wheel.

C = πd

C= π x 23

C = 72.3 cm (1 dp)

C = 2πr

C = 2 x π x 7.5

C = 47.1 cm (1 dp)

The Circumference of a Circle

Find the perimeter of the following semi-circles.

C = πd

C = 2πr

8 cm

1

9.5 cm

2

Perimeter = ½πd + d

= ½ x π x 8 + 8

= 20.6 cm (1 dp)

Perimeter = ½πd + d

= ½ x π x 9.5 + 9.5

= 24.4 cm (1 dp)

Perimeter = ¼(2πr) + 2r

= ¼ x 2 x π x 6 + 2 x 6

= 21.4 cm (1 dp)

Perimeter = ¾(2πr) + 2r

= ¾ x 2 x π x 8.5 + 2 x 8.5

= 57.1 cm (1 dp)

The Circumference of a Circle

Find the perimeter of the ¼ and ¾ circles.

C = πd

C = 2πr3 4

6 cm 8.5 cm

The Circumference of a Circle

Find the diameter/radius of the following circles.

C = πd

C = 2πr

C = 25 cm find the diameter.

5 6

πd = 25

d = 25/π

d = 8.0 cm (1 dp)

2πr = 30

r = 30/(2π)

r = 4.8 cm (1 dp)

C = 30 cm find the radius.