Now that we have moved up a level from key stage 3 to key stage 4, **algebra** becomes the most central part of mathematics, replacing **number** as being the topic where we will develop the most. Although, as you know, they are in many ways the same topic – algebra is a more general form of number, where instead of doing calculations with numbers, we do calculations with letters, knowing that each of these letters can be used to represent different numbers.

We combine our study of algebra with a study of graphs, or **cartesian geometry**, which is the special way in which we relate algebra to geometry. We will often alternate between talking about points with an x-coordinate and a y-coordinate and equations that relate x and y.