Question

how to tell if a graph is a function

Answer 1

?

Answer 2

You use the vertical line test. If you can draw a vertical line though the graph and it intersects it only once, it is a function. If the line crosses the graphs more than once it is not.

Answer 3

if it passes the "vertical line test"
no vertical line can cross the graph more than once

Answer 4

A graph represents a function if and only if every input generates a single output.

Answer 5

Here @satellite73 has shown what f(x) = yes looks like and what f(x) = no looks like. I have never heard of these functions before. He must be smart to know that.

Answer 6

for those who don't get it, it's a joke. fudge.