\documentclass{article}
\usepackage{amsmath}
\begin{document}
\section{section}
    change $\frac1a$
    
    \begin{subequations} % eq 1
        \begin{gather}
            \begin{align}
                a &= 1\\
                b &= 1
            \end{align}
        \end{gather}
    \end{subequations}
    \begin{equation} % eq 2
        a + b= 2
    \end{equation}
    \begin{equation} % eq 3
        \frac {a} {b}  = 1
    \end{equation}
        \begin{equation} % eq 4
        \frac   %Here is a fraction
        {c}  % Numerator
        {d}  % Denominator
        = 1
    \end{equation}
\end{document}

