# 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. ## 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. 