# Maths – Circle

The **circle** is a 2D (flat) shape that can be made by tracing a curve that is always the same distance from a point that we call the centre.

# Maths – Circle Facts

__Facts__

The **centre **of a circle is the point in the very middle.

The **radius **of a circle is a line from the centre of the circle to a point on the side. The radius is the same length all the way around the circle. Mathematicians use the letter *r *for the length of a circle’s radius.

The **diameter **of a circle is a straight line that goes from one side to the other, passing right through the centre of the circle. Diameter means ‘all the way across’. Mathematicians use the letter *d *for the length of this line.

The diameter of a circle is equal to twice its radius. (*d = 2 x r* **or ***d = 2r*)

The **circumference **of a circle is the distance all the way around it. Circumference means ‘all the way around’. Mathematicians use the letter *c *for the length of this line.

An **arc** is part of the circumference of a circle. The full arc of a circle measures 360 degrees.

A **chord **is a straight line joining two points on a circle. The diameter is an example of a chord.

A **semicircle **is a shape that forms half a circle.

Circles have a high level of **symmetry.**

**Good to know!**

The circle shape is a favourite of humans and can be seen in many designs.

The invention of the wheel (a circle shape) was one of the most important in human history.

## Learning resources

## Example questions

- Tell me the definition of a circle.
- What does ‘diameter’ mean?
- What is an arc?
- What is a chord?
- Tell me a fun fact about circles.
- What is the name of a triangle whose sides are all of equal length?
- What makes a triangle a scalene triangle?
- Tell me about the angles in an acute-angled triangle.
- What is the process of measuring using triangles called?
- Other than the circle and the triangle, name two geometric plane shapes.

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