Question

What does not represent a function?Y=4Y=x-4X^2+y^2=4Y=x^2-4

Answer 1

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:

$\begin{gathered} x^2+y^2=4 \\ y^2=4-x^2 \\ y=\pm\sqrt[]{4-x^2} \end{gathered}$

Thus, the third equation does not represent a function.