horizontally align same variables among equations in align

Question

I want to align same texts between equations, like ET, E, and T in this situation.

\documentclass[12pt]{article}
\usepackage{mathtools}

\begin{document}

\begin{align}
    ET &= E+ T &\\
    \delta_{ET} ET &= \delta_{E} E + \delta_{T} T &
    \label{eq:mass balance} 
\end{align}
\end{document}

the result is something like this

But I want it show up like this

Would anyone kindly give me a hint?

Answer 1

The typical approach here is to use \phantoms for the elements that are missing:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
  \phantom{\delta_{ET}}ET &= \phantom{\delta_E}E + \phantom{\delta_T}T \\
           \delta_{ET} ET &=          \delta_E E +          \delta_T T
\end{align}

\end{document}

You can also use alignat:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{alignat}{3}
              & ET ={} &          E +{} &&          T \\
  \delta_{ET} & ET ={} & \delta_E E +{} && \delta_T T
\end{alignat}

\end{document}

Alternatively, you can use other box manipulations like eqparbox:

\documentclass{article}

\usepackage{amsmath}

\usepackage{eqparbox,xparse}

% https://tex.stackexchange.com/a/34412/5764
\makeatletter
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother

\newcommand{\ET}[2][ET]{\eqmathbox[#1][r]{#2 ET}}
\newcommand{\E}[2][E]{\eqmathbox[#1][r]{#2 E}}
\newcommand{\T}[2][T]{\eqmathbox[#1][r]{#2 T}}

\begin{document}

\begin{align}
  \ET{}            &= \E{}         + \T{}         \\
  \ET{\delta_{ET}} &= \E{\delta_E} + \T{\delta_T}
\end{align}

\end{document}