Function y = x/(x-1) is self-inverse: x = y/(y-1) (symmetric on first diagonal y=x). It is the function giving the ratio y = XA/XB for a point on line AB described parametrically through. X = (1-x)A + xB. Then the ratio. y = XA/XB = x/(x-1). The graph of the function describes a rectangular hyperbola. The red points are the foci of the hyperbola. Point (1,1) is its center and x = 1, y = 1 are its asymptotic lines.