[math]x\left(Θ\right)=\left(R-r\right)cosΘ+dcos\left(\frac{\left(R-r\right)}{r}\right)Θ[/math][br][math]x\left(Θ\right)=\left(R-r\right)sinΘ+dsin\left(\frac{\left(R-r\right)}{r}\right)Θ[/math]