Functions can be really useful once you have managed to get your head round how they work. For GCSE, you will need to understand that fg(x) means you do g(x) first and then do f(g(x)). You will also need to understand inverse functions.

A function has a domain and a co-domain. The domain is the starting point and the co-domain is the finishing point. The function's job is to map the domain to the co-domain. A function can only map one point in the domain to one point in the co-domain. This means that squares and square roots cannot be functions as these map to two points in one or other of the domains. 5 times 5 is 25. -5 times -5 is 25.

Function Machines

Basic function machines going forward and backward.

Calculating graphs

Using a table to calculate the value of function y given points x.

Explanation of functions

Explanation of functions and domains with questions.