In your original equation x^2 - y^2 = 2xy divide both sides by y^2. You will get= , or = 0. In the last equation, introduce new variable t = . Then the equation takes the form t^2 - 2t - 1 = 0. Apply the quadratic formula: = = = . Since t = x/y and x > 0, y > 0, only positive root t = works and gives the Answer. = .