Answer
The equation does not define $y$ as a function of $x$.
Work Step by Step
Solve for $y$:
$y ^2 = 4-x^2$
$y=± \sqrt{4-x^2}$
This relation is s not a function, because a distinct x - value corresponds to two different y-values.
For example, (0, 2) and (0, –2)
