forVanya.tex

\documentclass{extreport}

% Математика
\usepackage{mathtools}

\begin{document}
$$
\begin{cases}
\frac{dy}{dx} 
=
f(x,y)
\\
y(x_0) 
=
y_0
\end{cases}
$$

$$
y
=
(y_0(x),...,y_n(x) )^T
$$

$$
f(x,y)
=
(f_0(x,y),...,f_n(x,y))^T
$$

\end{document}