Hypotrochoid

In geometry, a hypotrochoid is a roulette traced by a point attached to a circle of radius r rolling around the inside of a fixed circle of radius $R$, where the point is a distance d from the center of the interior circle. 

The parametric equations for a hypotrochoid are: $$\begin{array}{rl}& x(\theta )=(R-r)\cos \theta +d\cos \left(\frac{R-r}{r}\theta \right)\\ & y(\theta )=(R-r)\sin \theta -d\sin \left(\frac{R-r}{r}\theta \right)\end{array}$$ where $\theta$ is the angle formed by the horizontal and the center of the rolling circle (these are not polar equations because θ is not the polar angle). 

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