A limit is well-defined if and only if both the left-sided limits and the right-sided limits exist and are finite.
In this case, the right-sided limit exists and is 0, since as `x->0^+` , then the function approaches 0.
On the other hand, the left-sided limit does not exist. The reason it does not exist is because when `x<0`
even by a little bit, then the expression `x-x^2` is negative, which means we can't take the square root.
Since the left-sided limit does not exist, this means that the limit is not well-defined.