б)$$\frac{1}{10 \cdot 11} + \frac{1}{11 \cdot 12} + \frac{1}{12 \cdot 13} + ... + \frac{1}{18 \cdot 19} + \frac{1}{19 \cdot 20}$$;

$$\frac{1}{10 \cdot 11} + \frac{1}{11 \cdot 12} + \frac{1}{12 \cdot 13} + ... + \frac{1}{18 \cdot 19} + \frac{1}{19 \cdot 20} = (\frac{1}{10} - \frac{1}{11}) + (\frac{1}{11} - \frac{1}{12}) + (\frac{1}{12} - \frac{1}{13}) + ... + (\frac{1}{18} - \frac{1}{19}) + (\frac{1}{19} - \frac{1}{20}) = \frac{1}{10} - \frac{1}{20} = \frac{2}{20} - \frac{1}{20} = \frac{1}{20}$$ Ответ: $$\frac{1}{20}$$