126. \(\left(\frac{1}{y}-\frac{1}{x+y}\right):\frac{x}{y}\)

\(\frac{1}{y}-\frac{1}{x+y}=\frac{x}{y(x+y)}\).

\(\frac{x}{y(x+y)}:\frac{x}{y}=\frac{x}{y(x+y)}\cdot\frac{y}{x}=\frac{1}{x+y}\).

Ответ: \(\frac{1}{x+y}\).