Pomocí rozvoje
$$
{\frac {1}{\sqrt {1+{x}^{2}}}}=(1-{\frac {1}{2}}{x}
^{2}+{\frac {3}{8}}{x}^{4}-{\frac {5}{16}}{x}^{6}+O
\left ({x}^{8}\right ))
$$
a Abelovy vìty vypoètìte 
$$
1-{\frac {1}{2}}+{\frac {1.3}{2.4}}-{\frac {1.3.5}{2.4.6}}
+ \cdots ={\frac {\sqrt {2}}{2}} \quad .
$$


