function: Why the function define by only one value to the y?

[shahar]

Why the function define by only one value to the y?
What problems can it cause if I don't define function in this way?

 

[Dr.Peterson]

Why the function define by only one value to the y?
What problems can it cause if I don't define function in this way?

If a "function" has more than one value for a given input, how do you know which one to use?

The definition of a function is intended simply to make them useful for calculations.

There is in fact a concept of a multi-valued "function" in some areas of math; but they are very hard to work with. You wouldn't want to try.