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…

_{}

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…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:

_{}