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

\cyr{Pomo\'cna funkcija:}
$$\tilde f(x)=\cases
\pi,&0<x<\pi,\cr
-\pi,&-\pi<x<0,\cr
0,&x=0\;\vee\;x=\pi\;\vee\;x=-\pi,\cr
\endcases$$
\cyr{i }$\tilde f(x+2\pi)=\tilde f(x)$\cyr{ za }$x\in\R$. $\tilde f$\cyr{ je neparna }$\;\Longrightarrow\;\forall
n\in\N\cup\{0\}\quad a_n=0$.
\bye