\input amstex
\input cyracc.def
\font\tencyr=wncyr10
\def\cyr#1{{\tencyr\cyracc#1}}

$$b_n=\frac{1}{\pi}\int_{-\pi}^{\pi}\tilde f(x)\,\cos(nx)\,{\rm d}x=\frac{2}{n}-\frac{2\cos(n\pi)}{n}=\frac{2}{n}\bigl(1-(-1)^n\bigr)$$

$$S(x)=4\sum\limits_{k=0}^\infty\frac{\sin\bigl((2k+1)x\bigr)}
{2k+1},$$
\bye