Circles: calculate circumference, radius, and diameter
| The diameter of a circle is 10 millimetres. What is the circle's radius? |
|
 |
Use this formula to relate the radius and the diameter.
d = 2 x r
The diameter is equal to 10 millimetres. Replace d by 10 in the formula.
| d |
= 2 × r |
|
| 10 |
= 2 × r |
|
| 10 ÷ 2 |
= r |
|
| 5 |
= r |
The radius is 5 millimetres.
The circumference of a circle is the distance around the outside, just as the perimeter of a polygon is the distance around the outside. The circumference of a circle can be found by measuring its radius or diameter and the using the formula:
C = 2 × π × r (or C = π × d)
| The radius of a circle is 2 centimetres. What is the circle's circumference? |
|
 |
Use 3.14 for π.
Use this formula to find the circumference from the radius.
C = 2 × π × r
Use 3.14 as an approximation for π.
π ≈ 3.14
The radius is 2 centimetre. Replace r by 2 in the formula.
| C |
= |
2 × π × r |
|
| |
≈ |
2 × 3.14 ×2 |
|
| |
≈ |
12.56 |
The circumference is about 12.56 centimetres.
| The diameter of a circle is 8 metres. What is the circle's circumference? |
|
 |
Use 3.14 for π.
Use this formula to find the circumference from the diameter.
C = π × d
Use 3.14 as an approximation for π.
The diameter is 8 metres. Replace d by 8 in the formula.
| C |
= |
π × d |
|
| |
≈ |
3.14 ×8 |
|
| |
≈ |
25.12 |
The circumference is about 25.12 metres.
| The circumference of a circle is 12.56 metres. What is the circle's diameter? |
|
 |
Use 3.14 for π.
Use this formula to relate the diameter and the circumference.
C = π × d
Use 3.14 as an approximation for π.
π ≈ 3.14
The circumference is 12.56 metres. Replace C by 12.56 in the formula.
| C |
= π × d |
|
| 12.56 |
≈ 3.14 × d |
|
| 12.56 ÷ 3.14 |
≈ d |
|
| 4 |
≈ d |
The diameter is about 4 metres.
