Function defined by a relation in the form f(x) = \(\frac{b}{a}\) \(\sqrt{{a}^{2}~ – {x}^{2}}\) or f(x) = − \(\frac{b}{a}\) \(\sqrt{{a}^{2}~ – {x}^{2}}\) where a is the horizontal half-axis and b is the vertical half-axis of an ellipse centered on the origin point.