$\frac{x+y}{1/x+1/y} = $

$\frac{-x^2}{x^2-9} + \frac{x}{x+3}$

$\frac{x^3+8}{(x+2)^2} - \frac{x+4}{x+2}$

$(\frac{1}{x^2} - \frac{1}{y^2}) \div (\frac{1}{x} + \frac{1}{y}) = $

$\frac{\frac{3}{x} + 2}{\frac{5}{x^2} - \frac{1}{x}}$

$\frac{\frac{1}{a} - \frac{1}{b}}{\frac{1}{a} + \frac{1}{b}}$

$\frac{1}{x+2} - \frac{2x}{x^2-x-6}$