We often use formulae without even noticing. For example, we might convert miles to km by multiplying by 1.6, or find the circumference of a circle by multiplying pi by the diameter.

km = 1.6 x miles is an example of a formula.

We have already seen that km = 1.6 x miles and C = π × d are examples of formulae. There are many others that we use regularly in other subjects. Sometimes we have to construct our own formula:

A taxi firm charges £0.50 per mile plus a fixed charge of £2.00. Write down a formula for the cost (C) of hiring this taxi to travel 'n' miles.

- It costs £2 + £0.50 to travel 1 mile.
- It costs £2 + 2 x £0.50 to travel 2 miles.
- It costs £2 + 3 x £0.50 to travel 3 miles.

So travelling for 'n' miles will cost £2 + n x £0.50.

The formula is C = £2 + (n x £0.50).

Note: £0.50 = 50p.

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

**This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.**