you can just use elementary transformations. e.g. let delta=high-low and x=high/low then put a (0,1) constraint on delta and a (1, \infty) constraint on x. then you can solve for high and low and get low=delta/(x-1) and high=delta * x/(x-1). alternatively you can define y=x-1 and put a positivity constraint on y and write low=delta/y and high=delta * (y+1)/y