Equation Alignment With Steps

Question

I'm trying to show all of the steps for solving a linear equation like this:

\begin{aligned}
-4(6x - 5) & = 188 \\
(-4*6x)+(-4*-5) &  \\
-24x + 20 & = 188 \\
-20 & = -20 \\
-24x & = 168 \\
/-24 & = /-24 \\
x & = -7
\end{aligned}

This renders as I would expect it to, but if I remove the = sign from the -20 step (and /-24), the -20 no longer aligns correctly on the right side. Is there any way to have it still align without putting the equals sign in there? Thanks!

Answer 1

Do you mean something like this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}
-4(6x - 5) & = 188 \\
(-4*6x)+(-4*-5) &  \\
-24x + 20 & = 188 \\
-20 &\mathrel{\phantom{=}}  -20 \\
-24x & = 168 \\
/-24 & = /-24 \\
x & = -7
\end{aligned}
\end{equation}
\end{document}

I use the command phantom to tell LaTeX there is a equal sign and \mathrel for the correct sep.

Answer 2

if i understand correctly what you want to do, then i suggest replacing the line

-20 & = -20 \\

by

-20 & \phantom{{}={}} -20 \\

the extra braces around the = are needed to get the correct spacing since it's "hidden" from the adjacent characters that would otherwise force the correct spacing for a relation.

EDIT: as Werner points out, this gets the spacing wrong -- the minus becomes binary, not unary. to force a unary minus, you need

-20 & \phantom{{}={}} {-20} \\

Marco's answer is much nicer, and shows a sophisticated understanding of how to apply the symbol classes.