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

$I'(\alpha)$\cyr{ rachunamo smenom }$1-x^2=t^2$. \cyr{Dobija se }
$I'(\alpha)=-\frac{2}{\sqrt{1-\alpha^2}}\arctan\frac{\alpha}
{\sqrt{1-\alpha^2}}=-\frac{2}{\sqrt{1-\alpha^2}}\arcsin\alpha$, $\ldots$
\bye