13) \(\frac{20}{21} - \frac{7}{7}\)

Решение:

\(\frac{20}{21} - \frac{7}{7} = \frac{20}{21} - \frac{7 \cdot 3}{7 \cdot 3} = \frac{20}{21} - \frac{21}{21} = \frac{20-21}{21} = -\frac{1}{21}\)

Ответ: -\(\frac{1}{21}\)