[tex] (x^-^1+y^-^1)[x^-^2-(xy)^-^1+y^-^2]=( \frac{1}{x} + \frac{1}{y} )( \frac{1}{x^2} - \frac{1}{xy} + \frac{1}{y^2} )= \\ \\ = \frac{1}{x^3} - \frac{1}{x^2y} + \frac{1}{xy^2} + \frac{1}{x^2y} - \frac{1}{xy^2} + \frac{1}{y^3} = \frac{1}{x^3} + \frac{1}{y^3} [/tex]
