a = [tex]\frac{1}{1 \cdot 2} + \frac{1}{2 \cdot 3} + ... + \frac{1}{9 \cdot 10}[/tex]
0,8 < a < 1.
[tex]\it a = \frac{1}{1 \cdot 2} + \frac{1}{2 \cdot 3} + \frac{1}{3 \cdot 4} + \frac{1}{4 \cdot 5} ... + \frac{1}{9 \cdot 10} \\ \\ a = \frac{1}{1} - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \frac{1}{4} - \frac{1}{5} + ... + \frac{1}{9} - \frac{1}{10} \\ \\ a = ~ ^{10)} \frac{1}{1} - \frac{1}{10} \\ \\ a = \frac{10}{10} - \frac{1}{10} \\ \\ a = \frac{9}{10}[/tex]
a = 0,9
0,8 < 0,9 < 1.
