At any point of a curve (subtangent) × (subnormal) is equal to the square of the

slope of the tangent at that point slope of the normal at that point abscissa of that point square of ordinate of that point

square of ordinate of that point

We have, Sub normal = $y \frac{d y}{d x}$,  Sub tangent = $\frac{y}{\frac{d y}{d x}}$ ∴  Sub normal × Sub tangent = $y^2$