Missing $ inserted error when using \frac inside subequations environment?

Question

Here's a MNWE (minimum not working example):

\documentclass[12pt,letterpaper]{article}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

\section{Hello World}
    \begin{subequations}
        \frac{a}{b} = 0 \\
    \end{subequations}

\end{document}

The errors:

line 12: Missing $ inserted \frac{a}{b}
line 12: Extra }, or forgotten $ \frac{a}{b}
line 13: Missing $ inserted \end{subequations}
line 13: Missing } inserted \end{subequations}

How do I fix the MNWE?

Answer 1

subeequations does not in itself start math math mode, but all numbered equations within it will be numbered as 1a, 1b, etc. The environment can also contain text.

\documentclass[12pt,letterpaper]{article}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

\section{Hello World}
\begin{subequations}
Here is some text.
\begin{equation}
        \frac{a}{b} = 0
\end{equation}
Then follows some more text, followed by some more equations.
\begin{gather}
a=b \\
c = d
\end{gather}
\end{subequations}

The next equation is not part of the subequations.
\begin{equation}
g=h
\end{equation}
\end{document}