Subequations

수식 쓸때, numbering에 문자 추가하는 방법: subequations

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{subequations}

\label{equations}

\begin{align}

\label{eq:f}

f(x)&=ax + b \\

\label{eq:g}

g(x)&=cx^2 + dx + e

\end{align}

\end{subequations}

For example, see \eqref{equations}.

Or see equations \eqref{eq:f} and \eqref{eq:g}.

\end{document}

f(x) = ax+b (1a)

g(x) = cx^2+dx + e (1b)

http://tex.stackexchange.com/questions/38716/adding-letters-to-equation-numbers