The Pythagorean Identity

Concept

It is just x² + y² = 1

Since the unit circle point is (cos θ, sin θ) and lies on a radius-1 circle, cos²θ + sin²θ = 1 automatically.

Given one of the two and a quadrant, you can always recover the other: take the square root of 1 minus the square, then fix the sign from the quadrant.