Question

MEDAL!!!!

How can we create equations in two or more variables to represent relationships between quantities?

Answer 1

Context might be a little helpful. For example, a function implicity implies a certain type of relationship between the variables. However, certain functions can take different forms. For example, if a relationship between variables exists as a number of equalities, a parametric "set" of functions might describe how the variables relate using multiple functions of a single variable (for example, the parametric equation of a line in 3D space).

Answer 2

Or, it also sounds like you could be talking about modelling certain types of relationships... in which case, modelling the relationship between variables implies determining what the relationship is, then finding the function that best fits the data.