Circles

A circle is a plane figure formed by a curved and closed line. All points on the curved line are located at an equal distance from the centre of the circle (O).

Example:

The yellow and pink dots are at an equal distance from the centre of the circle.

The different parts of a circle can be described using specific vocabulary.

Here is a circle and its different parts.

The circumference is the measurement of the line that forms the circle’s contour.

Example:

The radius of a circle is a straight line (segment) connecting the centre of the circle (O) to any point on the circle’s contour.

Example:

The radius can be measured at any point on the circle.

Example:

This circle has a diameter of |10| centimetres (cm). What is the measurement of the radius?

The diameter of a circle is a straight line (segment) connecting two opposite points of the circle and passing through the centre (O).

Example:

The diameter can be measured at any point on the circle.

Example:

This circle has a radius of |6| centimetres (cm). What is the measurement of the diameter?

The central angle of a circle is an angle (|\angle|) formed by 2 radii and the vertex is located at the centre of the circle (O).

Example:

The central angle in the example is written |\angle|AOB.