A mathematical function is basically a way to state a relationship of variables. The idea of “the function (bx + c)” is basically to show what “bx + c” does on a mathematical basis. It’s an input-output kind of relationship. If you put 1 into the function for X, you might get 18. It’s like when you push the doorbell, and it makes the noise we all know as a doorbell’s ring. Often, the graph of a function has two “valleys” and a “hill” between them.
This is typically taught in Algebra II classes in the United States of America, something that people often deride as a class they won’t need unless they’re going into a math based field. (I was one of those people; I was wrong.)
