arcsin(y) = arccos(x) implies y = sin(arccos(x))

sin(arccos(x)) can be shown to equal ±sqrt(1-x²) with a simple triangle diagram and the pythagorean theorem, so we have y = ±sqrt(1-x²). This can be easily re-arranged to give x² + y² = 1.

sin^2 (x) + cos^2 (x) = 1