Let a and B be a set of nonempty numbers, and F: X → y be a corresponding rule from a to B. then the mapping f: a → B from a to B is called a function, denoted as y = f (x), where x ∈ a, y ∈ B, the original image set a is called the definition field of function f (x), and the image set C is called the value field of function f (x). Obviously, there is C ∈ B Why is it that the elements in the image set C ∈ B, not to say that B is the range of the function f (x), can remain?

Let a and B be a set of nonempty numbers, and F: X → y be a corresponding rule from a to B. then the mapping f: a → B from a to B is called a function, denoted as y = f (x), where x ∈ a, y ∈ B, the original image set a is called the definition field of function f (x), and the image set C is called the value field of function f (x). Obviously, there is C ∈ B Why is it that the elements in the image set C ∈ B, not to say that B is the range of the function f (x), can remain?

According to the definition of the function, as long as any x can find a unique value corresponding to it in B. there is no saying about B. It's OK when C = B