# Referencing an equation in align mode

Let say I have an labeled-equation like,

And I want to refer this equation in align mode like,

I want to know how can this be done using label and ref tools in latex? Any help would be appreciated. Thanks

The same way as you reference in normal text:

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}

$$\label{eq:abc} a = b + c$$
%
\begin{align*}
e + f &= g \\
\overset{\text{by \eqref{eq:abc}}}{\Longrightarrow} m + n &= p
\end{align*}

\end{document}


Like this?

Edit: I forgot on "by" ... corrected now

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{align}
a+b & = c       \label{eq:1}  \\
\xRightarrow{\text by \eqref{eq:1} }} m+n & =p
\end{align}
\end{document}


Hope the below coding solved your problem:

\documentclass{book}

\usepackage{amsmath,mathtools}

\begin{document}

\begin{align}

a=b+c\label{eq1}

\end{align}

\begin{align}

e+f&=g\label{eq2}\\

\xRightarrow{\text{by \eqref{eq1}}}m+n&=p

\end{align}

\end{document}

• amsmath is loaded by mathtools, so it is not necessary to load it again ... – Zarko Apr 18 '17 at 12:40
• blank lines aren't allowed in math mode. (perhaps this is just an accident of how this was input, but it's something you should rmember.) – barbara beeton Apr 19 '17 at 1:06