A question about spheres CCD

[lchut]

I'm trying to understand the sphere continues collsion detection algorithm defined by cloth::SwCollision<T4f>::collideSpheres function in SwCollision.cpp. And I’m confused by the definition of the variable rMin, which was defined as rMin = prevRadius + halfB * minusA * (curRadius - prevRadius). I don't understand what "Min" means and why (-ab/2) is chosen as interpolation factor. Combined with the context of the code, my understanding is that the interpolation factor should be -b/(2a). I can't solve my doubt from the formula derivation in the official document. I hope for your reply.