1) (-12,4+8,9)·1 7 2) (2 3 8 -1 5 6 ):( -1 5 8 )

Решение:

1. \( (-12,4+8,9) \cdot \frac{3}{7} = -3,5 \cdot \frac{3}{7} = -\frac{35}{10} \cdot \frac{3}{7} = -\frac{5}{10} \cdot 3 = -0,5 \cdot 3 = -1,5 \)
2. \( \left( \frac{3}{8} - 1\frac{5}{6} \right) : \left( -1\frac{5}{8} \right) = \left( \frac{3}{8} - \frac{11}{6} \right) : \left( -\frac{13}{8} \right) = \left( \frac{9 - 44}{24} \right) : \left( -\frac{13}{8} \right) = \left( -\frac{35}{24} \right) : \left( -\frac{13}{8} \right) = \frac{35}{24} \cdot \frac{8}{13} = \frac{35}{3 \cdot 13} = \frac{35}{39} \)

Ответ: 1) -1,5; 2) 35/39.