# Equation of Tractrix/Parametric Form

## Definition

Let $S$ be a cord of length $a$ situated as a (straight) line segment whose endpoints are $P$ and $T$.

Let $S$ be aligned along the $x$-axis of a cartesian plane with $T$ at the origin and $P$ therefore at the point $\tuple {a, 0}$.

Let $T$ be dragged along the $y$-axis.

The equation of the tractrix along which $P$ travels can be expressed in parametric form as:

$x = a \sin \theta$
$y = a \paren {\ln \cot \dfrac \theta 2 - \cos \theta}$

## Proof

Consider $P$ when it is at the point $\tuple {x, y}$.

The cord $S$ is tangent to the locus of $P$.

## Linguistic Note

The word tractrix derives from the Latin traho (trahere, traxi, tractum) meaning to pull or to drag.

The plural is tractrices.