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

$$f'(x)=\cases
6x^2+3x^2\sin\frac{1}{x^2}-2\cos\frac{1}{x^2},&x\neq 0,\cr
0,&x=0.
\endcases$$
\bye