please i want to know how did they compute the distance r' between these two non-concentric circles. you find enclosed here the scheme that represent the displacement of the two circles.

r'=sqrt(alpha)*beta+-sqrt(alpha*beta^2-alpha-rI^2)

where,

alpha=ox^2+oy^2

beta=cos(Pi-arctan(oy/ox)-theta)

and rI is the radius of the outer circle.

thanks an advance