Daniel B. answered 02/17/21
A retired computer professional to teach math, physics
The level curve is the set of points (x, y) satisfying
2x² + 2y² = 1
Rewriting
y² = 0.5 - x²
When x∈(-√0.5, √0.5) then y²>0 and therefore y is real and can be both positive and negative.
Therefore the answer of the last one
y = ±√(0.5 - x²)