For an expression to be a function, it has to have only one y value for each x value.
Using this, we can see that the first expression obeys it, and so it can be considered a function, even though it has no x.
The same can be said about the second and the fourth.
But the third can have two possible values for each x:
[tex]\begin{gathered} x^2+y^2=4 \\ y^2=4-x^2 \\ y=\pm\sqrt[]{4-x^2} \end{gathered}[/tex]Thus, the third equation does not represent a function.
