Here's the official definition:
rule is a function if
Check out this guy:
Let's graph those points:
is NOT a function since one element in the
goes to two elements in the range
(2 and 4.)
Or, you can just say that R is not a function since 1 goes to 2 AND 1 goes to 4.
So, it fails the vertical line test!
* put the letters along the bottom since they are the domain guys!
Is this a function?
* now the numbers go along the bottom!