# Length of Arc of Cycloid/Proof 2

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## Contents

## Theorem

Let $C$ be a cycloid generated by the equations:

- $x = a \left({\theta - \sin \theta}\right)$
- $y = a \left({1 - \cos \theta}\right)$

Then the length of one arc of the cycloid is $8 a$.

## Proof

Consider the tangent $PQ$ to both the generating circle and the cycloid itself.

By

:

- $PR = 2 PQ$

In the limit, where $P$ is at the cusp, the tangent $PQ$ is perpendicular to the straight line along which the generating circle rolls.

At this point:

- $PQ = 2 a$.

Thus at this point:

- $PR = 4 a$

But $4 a$ is half the length of one arc of $C$.

Hence the result.

## Historical Note

The geometric proof of the length of the arc of a cycloid was demonstrated by Christopher Wren in $1658$.

## Sources

- 1992: George F. Simmons:
*Calculus Gems*... (previous) ... (next): Chapter $\text {A}.15$: Torricelli ($1608$ – $1647$)