documentclass{book}
usepackage{standalone}
usepackage{mystyle}

begin{document}
input{./tex/dev.tex}
end{document}

documentclass{standalone}
usepackage{../mystyle}

usepackage[showframe]{geometry}

begin{document}

begin{multicols}{2}
begin{align}
    f(x) &= x^2+uv+u^3xy
end{align}
end{multicols}
end{document}

ProvidesPackage{mystyle}

usepackage[utf8]{inputenc}
usepackage{amsmath}
usepackage{mathrsfs}
usepackage{amsfonts}
usepackage{tikz}
usepackage{multicol}

newcommand{Tau}{mathcal{T}}
renewcommand{r}{mathbb{R}}
newcommand{q}{mathbb{Q}}