Multiplicative relationships in circles | Maths | Quizalize & Oak National Academy

Starter quiz

Which part of the circle is labelled?
- arc
- circumference
- diameter
- radius ✓
The ______ of a circle is the perimeter of the circle.
- 'circumference' ✓
If a circle has a radius of 12 cm then its diameter will be ______ cm.
- '24' ✓
Which symbol represents the ratio between the diameter and circumference of a circle?
- $\alpha$
- $\beta$
- $\Delta$
- $\pi$ ✓
Which are the first three digits of $\pi$ ?
- '3.14' ✓
A circle with diameter 10 cm has a circumference of ______ cm.
- 5 $\pi$
- 10 $\pi$ ✓
- 20 $\pi$

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There is a multiplicative relationship between the radius and diameter of a circle.
The ratio between the diameter and circumference of a circle is always the same.
That ratio is called π and it is an irrational number.
The formula for the circumference of a circle is C = πd.
Using the definition of the diameter you can also write this as C = 2πr.

Pupils may think that the π symbol represents a variable, similar to letters such as r and d.

Reiterate the difference between variables and constants. Explain that while the radius may differ between circles, π is always the same.

Diameter - The diameter of a circle is any line segment that starts and ends on the edge of the circle and passes through the centre.
Radius - The radius of a circle is any line segment that joins the centre of a circle to its edge.
Circumference - The circumference of a circle is the perimeter of the circle.