Functional equation

A functional equation is like a regular math equation, but instead of having two sides, it only has one side. It usually looks like this:

f(x) = Something

Functional equations are used to find the value of a "function" (f) when given a certain input (x). For example, if we have this equation:

f(x) = x + 3

we can find out what f(5) is by plugging in the number 5 for x, which would give us f(5) = 8.