\input amstex
\input cyracc.def
\font\tencyr=wncyr10
\def\cyr#1{{\tencyr\cyracc#1}}
\def\imag{\mathrm{Im}\,}
\def\logici{\;\wedge\;}

$$\cases
-\frac{2,\arcsin\alpha}{\sqrt{1-\alpha^2}},&\imag\frac{1}{\alpha^2}=0
\logici\alpha^2>0\logici\alpha^2>1,\cr
\hbox{\cyr{ne postoji}},&\hbox{\cyr{inache.}}
\endcases$$

\bye