Epicycloid (The Abscissa of a Point) Formula

Equation for calculate epicycloid (the abscissa of a point) is,

x = ( R + r ) · cos θ - r · cos ⁡ ( θ + a )

where,
x = The abscissa of the point
R = Radius of the larger circle
r = Radius of the smaller circle
θ = Tthe radian from the starting point to the tangential point
a = The radian from the tangential point to the moving point