# Hypocycloid ( parametric equation X- coordinate)

## Description

A hypocycloid is a special plane curve generated by the trace of a fixed point on a small circle that rolls within a larger circle. It is comparable to the cycloid but instead of the circle rolling along a line, it rolls within a circle. If the smaller circle has radius r, and the larger circle has radius R = kr, then the shape of the curve is depended on the value of k. If k is an integer, then the curve is closed, and has k cusps. Specially for k=2 the curve is a straight line and the circles are called Cardano circles.

If k is a rational number,say k = p/q, then the curve has p cusps.

If k is an irrational number, then the curve never closes, and fills the space between the larger circle and a circle of radius R − 2r.

Each hypocycloid (for any value of r) is a brachistochrone for the gravitational potential inside a homogeneous sphere of radius R.

## Variables

x_θ | X-coordinate (m) |

r | The radius of the small circle (m) |

k | Number of times the r (dimensionless) |

θ | Parameter ( 0 - 2π) (radians) |