Circumference & Area of Circles

Unit 5-3

Circumference

Formula for Circumference:

2C r** r is the radius **** 2r = d. d is the diameter. **

**Circumference is the perimeter of the circle. **

Circumference

6 cmAlways write out the formula to start with C = 2πr

Check if you have radius or diameter r = 6

Substitute into the formula

C = 2π(6)

C = 12π

or

C = 37.70

Don’t forget units!

cm

cm

Circumference

7.6 cm

C = 2πr

d = 7.6

C = 2π(3.8)

C = 7.6π

or

C = 23.88 cm

cm

r = 3.8

YOUR TURN -Calculate the perimeter of these circles.

4 cm

8.2 in

7 ft

11 cm

25.13 cm 25.76 in

34.56 cm

43.98 ft

Circumference

Given the Circumference, find the radius and diameter.

C = 25 ft

2C r25 2 r25

2r

3.98 ft r 7.96 ft d

YOUR TURN

Given the Circumference, find the radius and diameter.

37 .C in

5.89 .r in

11.78 .d in

Area

Formula for Area:

2A r** r is the radius **

Area

6 cmAlways write out the formula to start with A = πr2

Check if you have radius or diameter r = 6

Substitute into the formula

A = π(6)2

A = 36π

or

A =113.10

Don’t forget units!

cm2

cm2

Area

7.6 cm

A = πr2

d = 7.6

A = π(3.8)2

A = 14.44π

or

A = 45.36 cm2

cm2

r = 3.8

YOUR TURN -Calculate the area of these circles.

4 cm

8.2 in

7 ft

11 cm

50.27 cm2 52.81 in2

95.03 cm2

153.94 ft2

Area

Given the Area, find the radius and diameter.

A = 75 ft2

2A r275 r

275r

4.89 ft r

75r

9.77 ft d

YOUR TURN

Given the Area, find the radius and diameter.

2100 .A in

5.64 .r in

11.28 .d in