1.
Solution of differential equation x2=1+(xy)1dydx+(xy)2(dydx)22!+x^2 = 1 + \left(\frac{x}{y}\right)^{-1}\frac{dy}{dx} + \frac{\left(\frac{x}{y}\right)^{-2}\left(\frac{dy}{dx}\right)^2}{2!} + \ldots is