X is called an "independent variable"

F(x) is called "function"

How to understand: x gets f(x) through the correspondence of f.

For example, if an F is "the square of a certain number minus one", then you can get 24 (F 5(x=5) = 24) by adding a 5.

But the expression of words is very verbose, so we use X instead of "a certain number"

Replace "required quantity" with f(x) and f (x) = x2- 1.

The rule of "the square of a certain number minus one"

Sometimes we don't need to know the expression of the corresponding law, we just need to express that there is such a function, so we can just write f(x).

You can give another excessive example: f: "someone's father", then you can get "Bush Sr." by typing "Bush Jr.", right?

But this f can't be written as f(x). We call this correspondence that is not a number "mapping".