\documentclass[12pt,twoside]{article}
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\usepackage[english,russian]{babel}
\usepackage[T2A]{fontenc}
\RequirePackage[discret]{correct}
\textwidth=170mm
\textheight=257mm
\oddsidemargin=4.6mm
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   ‹Ґў п (\langlemy) Ё Їа ў п (\ranglemy) гЈ«®ўлҐ бЄ®ЎЄЁ.
%   ‡­ ЄЁ Ї а ««Ґ«®Ја ¬¬  (\parallelogram), ваҐгЈ®«м­®© ЇЁа ¬Ё¤л 
%   (\tetrahedron) Ё Ї а ««Ґ«ҐЇЁЇҐ¤  (\parallelepiped).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\font \kostinten = kostin at 12pt
\font \kostinseven = kostin at 9pt
\font \kostinfive = kostin at 6pt
\newfam \kostinfam
\edef\kostinfamhex{\ifcase\kostinfam 0\or 1\or 2\or 3\or 4\or 5\or 6\or 7\or
         8\or 9\or A\or B\or C\or D\or E\or F\fi}

\textfont \kostinfam \kostinten
\scriptfont \kostinfam \kostinseven
\scriptscriptfont \kostinfam \kostinfive

\mathchardef \langlemy "\kostinfamhex 31
\mathchardef \ranglemy "\kostinfamhex 32
\mathchardef \parallelogram "\kostinfamhex 33
\mathchardef \tetrahedron "\kostinfamhex 34
\mathchardef \parallelepiped "\kostinfamhex 35

\begin{document}

\normalsize
\parindent=7mm
               
{\bf Задача.} Найти расстояние от точки $P = \langlemy 2,\;4,\;3\ranglemy$ 
до плоскости $Q = \{ 3x-5y+z-8=0 \}$. 

$\triangle ABC$, $S_{\triangle}(\vec{a},\;\vec{b})$

$\parallelogram ABCD$, $S_{\parallelogram}(\vec{a},\;\vec{b})$

$\tetrahedron ABCD$, $V_{\tetrahedron}(\vec{a},\;\vec{b},\;\vec{c})$

$\parallelepiped ABCDA'B'C'D'$, $V_{\parallelepiped}(\vec{a},\; \vec{b},\;
\vec{c})$

$\langlemy]$, $\langlemy\}$, $[\ranglemy$, $\{\ranglemy$, 
${=}\langlemy$, $\ranglemy{=}$ 

$\triangle$, $\tetrahedron$, $\triangle$

$\triangle$, $\parallelogram$, $\triangle$

$\triangle$, $\parallelepiped$, $\triangle$

\end{document}
