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:
