Tangent to Cycloid is Vertical at Cusps
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Theorem
The tangent to the cycloid whose locus is given by:
- $x = a \paren {\theta - \sin \theta}$
- $y = a \paren {1 - \cos \theta}$
Proof
From Slope of Tangent to Cycloid, the slope of the tangent to $C$ at the point $\tuple {x, y}$ is given by:
- $\dfrac {\d y} {\d x} = \cot \dfrac \theta 2$
At the cusps, $\theta = 2 n \pi$ for $n \in \Z$.
Thus at the cusps, the slope of the tangent to $C$ is $\cot n \pi$.
From Shape of Cotangent Function:
- $\ds \lim_{\theta \mathop \to n \pi} \cot \theta \to \infty$
Hence the result by definition of vertical tangent line.
$\blacksquare$
Sources
- 1992: George F. Simmons: Calculus Gems ... (previous) ... (next): Chapter $\text {B}.21$: The Cycloid