Simplified form of 1/y-1/x divided by 1/y+1/x
\(\frac{ \frac{1}{y} - \frac{1}{x} }{ \frac{1}{y} + \frac{1}{x} } = \frac{ \frac{x-y}{xy} }{ \frac{x+y}{xy} } = \frac{x-y}{x+y}\)
