Area & Circumference of Circles

Area & Circumferencevideo

Area & Circumference

What are the properties of a circle?

• A circle is a shape that is made up of all the points on a 2D plane that are equidistant from a single point

  • Equidistant means the same distance

• The circumference of a circle is its perimeter

• The diameter, d, of a circle is twice its radius, r

• $\pi$ (pi) is the number (3.14159 …) that is the ratio between a circle’s diameter and its circumference

  • You may be asked to give an answer to a question as an 'exact value' or 'in terms of $\pi$'

What circle formulae do I need to know?

• The formulae for the area and circumference of a circle are given to you in your exam

  • However, it can be useful to remember them!

• The area of a circle can be calculated using the formula:

  • $A=\pi r^{\mathit{2}}$

• The circumference of a circle can be calculated using either of the following two formulae:

  • $\mathrm{C}=\pi \mathit{ }d$

  • $C=2\pi \mathit{ }r$ 

• Working with circle formulae is just like working with any other formula:

  • Write down what you know (or what you want to know)

  • Pick the correct formula

  • Substitute the values in and solve

How do I find the circumference of a circle?

• Identify the diameter

  • This is double the length of the radius

• Multiply the diameter by π

How do I find the area of a circle?

• Identify the radius

  • This is half the length of the diameter

• Square the radius

• Multiply the radius squared by π