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

\noindent
\cyr{Napisati Furijeov red funkcije }$f(x)=\pi\,\sgn\sin x$.
\cyr{ Izrachunati }\fbox{$\sum\limits_{n=0}^\infty\frac{(-1)^n}{4n+1}$}
\cyr{ i }$\sum\limits_{n=0}^\infty\frac{1}{(2n+1)^2}$.
\bye