I have two questions of the sort. The first one is x^2 + y^2 = 4 and I reorganized to look like y =(-x^2 +)^1/2. At first, I sensed it to be a function, but I was found to be wrong. Can someone explain why it is not a function, the same goes for the second equation y^2 = x^2 - 1, which I change into y = (x^2 - 1)^1/2. Thank you.