13) 1.4 / (1/7 - 1) =

13)

$$\frac{1.4}{\frac{1}{7} - 1} = \frac{1.4}{\frac{1}{7} - \frac{7}{7}} = \frac{1.4}{\frac{1-7}{7}} = \frac{1.4}{\frac{-6}{7}} = 1.4 : (\frac{-6}{7}) = 1.4 \cdot (\frac{-7}{6}) = \frac{1.4 \cdot (-7)}{6} = \frac{-9.8}{6} = -1.63$$

Ответ: $$-1.63$$