18. $$(\frac{1}{9})^2 - 20 \cdot \frac{1}{9}$$

Решение:

1. $$(\frac{1}{9})^2 = \frac{1}{81}$$
2. $$20 \cdot \frac{1}{9} = \frac{20}{9}$$
3. $$\frac{1}{81} - \frac{20}{9} = \frac{1}{81} - \frac{20 \cdot 9}{9 \cdot 9} = \frac{1}{81} - \frac{180}{81} = -\frac{179}{81}$$

Ответ: