Given \( y = \sin x \), the slope of tangent = \( \cos x \) ⇒ slope of normal = \( -1/\cos x \)
Use normal line equation:
\[
y - \sin x = -\frac{1}{\cos x}(x - x) \Rightarrow \text{passes through origin ⇒ substitute (0,0)}
\Rightarrow -\sin x = \frac{x}{\cos x}
\Rightarrow x^2 = \sin^2 x - \sin^4 x \Rightarrow \boxed{x^2 = y^2 - y^4}
\]