Parametric derivative

Definition

Algebraic definition

The parametric derivative $dy/dx$ for a parametric curve $x = f(t), y = g(t)$ at a point $t = t_0$ is given as:

$\! \frac{dy}{dx} = \frac{g'(t_0)}{f'(t_0)}$

As a general function of $t$, the parametric derivative $dy/dx$ is defined as $g'(t)/f(t)$.

MORE ON THE WAY THIS DEFINITION OR FACT IS PRESENTED: We first present the version that deals with a specific point (typically with a $\{ \}_0$ subscript) in the domain of the relevant functions, and then discuss the version that deals with a point that is free to move in the domain, by dropping the subscript. Why do we do this?
The purpose of the specific point version is to emphasize that the point is fixed for the duration of the definition, i.e., it does not move around while we are defining the construct or applying the fact. However, the definition or fact applies not just for a single point but for all points satisfying certain criteria, and thus we can get further interesting perspectives on it by varying the point we are considering. This is the purpose of the second, generic point version.