\(\pi\) or **pi** is a Greek letter used to represent a mathematical constant. It is equal to the ratio of a circle’s circumference to its diameter. It approximate value is 3.14159, but as it is an irrational number, its decimal expansion goes on forever without a recurring pattern.

\(\pi\) turns out to be very useful in a number of areas of mathematics, notably in geometry. AS well as it applications to circles, it is used in formulas relating to volumes and surface areas of spheres and cones, and it appears in higher trigonometry too.

**Relevant lessons:**

- N8a – Calculating exactly with fractions and with multiples of π 3-5 Pi
- G17b – Circumference of a circle
- G17c – Area of a circle
- G17e – Perimeter and area of composite shapes made up of polygons and sectors of circles, diameters
- G17f – Surface area and volume of spheres, pyramids, cones and composite solid
- G18a – Arcs and sectors of circles