How to align a system of numbered equations

Question

There are solutions to aligning a system unnumbered system of linear equations here: Aligning system of equations with zero coefficients I want a similar solution to numbered system of linear equations. I have also posted a question on using syteme for this here: How to do cross reference in systeme package? However, I find the cross-reference in systeme too painful to use. How to align a system of linear equations with other packages with the standard latex cross reference interface like alignat?

Do you need/want a curly bracket (or brace) to encompass the system of equations? — Werner, Feb 12 '21 at 06:02
I don't want curly brackets. I want the equations separately numbered so that I could use cross reference. — S. Venkataraman, Feb 12 '21 at 06:06

score 2 · Accepted Answer · answered Feb 12 '21 at 20:34

It's fairly straight forward to put a system of linear equations in an alignat if you consider the following basic setup:

Each alignment point & has a right-&-left (or r&l) alignment structure;
Keep variables, operators and relations in their own "column";
Add {} around operators/relations to ensure proper spacing (as in {}+{}, {}-{} and {}={});
x variables requires x+1 alignment points (so, for 3 variables, you need \begin{alignat}{4}).

In the examples below I've marked the right-alignment point with numbers, which translates to the value used with alignat:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
% 3 variables ~ 4 alignment points
\begin{alignat}{4}
%  1         |  2         |  3         |  4
  2x & {}+{} &  y & {}-{} & 2z & {}={} &  3  \
   x & {}-{} &  y & {}-{} &  z & {}={} &  0  \
   x & {}+{} &  y & {}+{} & 3z & {}={} & 12
\end{alignat}
% 3 variables ~ 4 alignment points
\begin{alignat}{4}
%  1         |  2         |  3         |  4
   x &       &    & {}+{} &  z & {}={} &  6  \
     & {}-{} & 3y & {}+{} &  z & {}={} &  7  \
  2x & {}+{} &  y & {}+{} & 3z & {}={} & 15
\end{alignat}
% 4 variables ~ 5 alignment points
\begin{alignat}{5}
%   1         |  2         |  3         |  4         |  5
    x & {}+{} &  y & {}+{} &  z & {}+{} &  w & {}={} & 13  \
   2x & {}+{} & 3y &       &    & {}-{} &  w & {}={} & -1  \
  -3x & {}+{} & 4y & {}-{} &  z & {}+{} & 2w & {}={} & 10  \
    x & {}+{} & 2y & {}-{} &  z & {}+{} &  w & {}={} &  1
\end{alignat}
\end{document}

With the above structure in place, you can \label-\ref whatever you need in the usual way.

How to align a system of numbered equations

1 Answers1