2) 5/6+(-5/7);

2) $$\frac{5}{6} + (-\frac{5}{7}) = \frac{5}{6} - \frac{5}{7} = \frac{5 \cdot 7}{6 \cdot 7} - \frac{5 \cdot 6}{7 \cdot 6} = \frac{35}{42} - \frac{30}{42} = \frac{35 - 30}{42} = \frac{5}{42}$$

Ответ: $$\frac{5}{42}$$