1 1 --- + --- x y _________ 1 1 --- -- --- x y

[tex] \frac{ \frac{1}{x} + \frac{1}{y} }{ \frac{1}{x} - \frac{1}{y} } = \frac{ \frac{y + x}{xy} }{ \frac{y - x}{xy} } = \frac{y + x}{y - x} [/tex]
semoga membantu