$$\operatorname{tg} x = \sum\limits_{n = 1}^{ + \infty } {\left| {{B_{2n}}} \right|\frac{{{2^{2n}} - 1}}{{\left( {2n} \right)!}}{x^{2n - 1}},{\text{ }}\left| x \right| < \pi /2} $$