When one variable is directly related to another, you can use the proportionality symbol to describe the relationship.  The symbol is a ‘’.  So if we write something like…

…what we mean is that the value of ‘a’ is proportional to the value of ‘b’, and that we can write a quantitative (one involving actual numbers and values) relationship using just a constant of proportionality, like this:

Same goes for ‘proportional to the square’ relationships, for instance for , we would write that y is proportional to the square of x: