\documentclass[a4paper,12pt]{article}
\usepackage[russian]{babel}
\usepackage[utf8]{inputenc}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{enumerate}
\usepackage{titlesec}
\usepackage[left=20mm, top=15mm, right=15mm, bottom=15mm, nohead, footskip=10mm]{geometry} % настройки полей документа
\usepackage{indentfirst}

\newcommand{\sectionbreak}{\clearpage}

\renewcommand{\emptyset}{\varnothing}
\renewcommand{\bar}[1]{\overline{#1}}

\parindent=1.25cm