Parametric Equation of Involute of Circle

Theorem
Let $C$ be a circle of radius $a$ whose center is at the origin of a cartesian plane.

The involute $V$ of $C$ can be described by the parametric equation:


 * $\begin {cases} x = a \paren {\cos \theta + \theta \sin \theta} \\ y = a \paren {\sin \theta - \theta \cos \theta} \end {cases}$

Proof
By definition the involute of $C$ is described by the endpoint of a string unwinding from $C$.

Let that endpoint start at $\tuple {a, 0}$ on the circumference of $C$.


 * Involute-of-Circle.png

Let $P = \tuple {x, y}$ be an arbitrary point on $V$.

Let $Q$ be the point at which the cord is tangent to $C$.

Then $PQ$ equals the arc of $C$ from which the cord has unwound.

Thus:
 * $PQ = a \theta$

where $\theta$ is the angle of $OQ$ to the $x$-axis.

Thus:

Hence the result.