So far we have played with various expressions in Maxima, Maple and in Mathematica, discovering in the process that the three are pretty similar, even if there are subtle differences between them - with Maxima and Maple being perhaps closer to each other than to Mathematica, not in the least because of a common heritage, that of Macsyma.
We have also discovered how we can assign numerical or symbolic expressions to variables. Now we make another step and introduce a concept of a function.