When the radius of a circle is known, the are of the circle can be calculated using a formula.

The formula is `A = pir^2`, where π is 3.14 and *r* is the radius of the circle. π is a fixed value and can be found on a button on your calculator.

If the diameter is known, the radius can be found by halving the diameter.

The area is an area measurement, and the answer will be in cm^{2}, m^{2}, etc.

What is the area of the circle, below?

Area of a circle | A | `= pir^2` |

substitute | `= pi xx 15^2` | |

`= 706.5` |

Answer: 706.5 cm^{2}

What is the diameter of a wheel, if the area is 500 cm^{2}? Give your answer to the nearest centimetre.

Area of a circle | A | `= pir^2` |

substitute | 500 | `= pi xx r^2` |

divide both sides by π | 159.24 | `= r^2` |

square root both sides | 12.62 | `= r` |

diameter = 2 x radius | 12.62 x 2 | `= d` |

25.24 | `= d` | |

nearest cm | 25 | `= d` |

Answer: 25 cm

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