Now, we want this expression to be less than . Since we have three terms, it suffices to make sure each term is less than , so that their sum would be less than . Hence we find that:
And it seems straight-forward to make and , so that .
But there is a problem with (1) and (2), what if or are zero? Big trouble. So we can throw in an extra 1 in there to ensure that division by zero never happens. At the same time, increasing the denominator makes the fraction smaller, and keeps the expression positive, thus it is good all around for an upper bound. Got it?