How to align math equations even when some variables are missing? - latex

Question

I want the variables and signs aligned in a set of equations.
Desired output:
What I am trying in Mathjax:
\[
\begin{align}
2&x_1 - x_2 &+ 1.5&x_3 &= 8 \\
&x_1 &- 4&x_3 &= -1
\end{align}
\]
What I got:
So, what do I have to do to have the same alignment as in the "desired output" image?

Answer 1

There are a number of ways to achieve this. Below I use an array (with appropriate stretch and spacing), alignat and align coupled with eqparbox for measuring similarly-tagged boxes (this latter approach requires two compilations with every change in the largest element associated with every <tag>):
\documentclass{article}
\usepackage{amsmath,eqparbox}
%\usepackage{xparse}% If you have LaTeX2e < 2020-10-01
% https://tex.stackexchange.com/a/34412/5764
\makeatletter
% \eqmathbox[<tag>][<align>]{<math>}
\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
\begin{document}
\[
\renewcommand{\arraystretch}{1.2}
\setlength{\arraycolsep}{0pt}
\begin{array}{ r c r c r c r }
2 x_1 & {}-{} & x_2 & {}+{} & 1.5 x_3 & {}={} & 8 \\
x_1 & & & {}-{} & 4 x_3 & {}={} & -7
\end{array}
\]
\begin{alignat*}{4}
2 x_1 & {}-{} & x_2 & {}+{} & 1.5 x_3 = {} && 8 \\
x_1 & & & {}-{} & 4 x_3 = {} && -7
\end{alignat*}
\begin{align*}
\eqmathbox[x1][r]{2 x_1} - \eqmathbox[x2][r]{x_2} + \eqmathbox[x3][r]{1.5 x_3} &= \eqmathbox[c][r]{8} \\
\eqmathbox[x1][r]{ x_1} \phantom{{}-{}} \eqmathbox[x2][r]{} - \eqmathbox[x3][r]{4 x_3} &= \eqmathbox[c][r]{-7}
\end{align*}
\end{document}
All yield similar output: