How do I align a system of equations inside square brackets, for different cases?

Question

I thought maybe fractions without fraction bar, but seems inconvenient. Also, how would I extend this for more than 2 lines?

Please add a compilable code that produces the unwanted result, including the packages used. — Simon Dispa, Mar 28 '22 at 19:19
I'd suggest editing the title of this question so it is more useful when searched, or just informative to the reader. — frabjous, Mar 29 '22 at 02:57

score 6 · Accepted Answer · answered Mar 28 '22 at 20:22

6

You can do this with \aligned:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
[
\left[\begin{aligned}&x^2+2x-3(x+1)+3=0,&x+1\ge0\ &x^2+2x-3(-x-1)+3=0,&x+1<0\end{aligned}\right.\Rightarrow
\left[\begin{aligned}&x^2-x=0,&x\ge-1\ &x^2+5x+6=0,&x<-1\end{aligned}\right.\Rightarrow
\left[\begin{aligned}&x=1,x=0\&x=-2,x=-3\end{aligned}\right.
]
\end{document}

answered Mar 28 '22 at 20:22

Sandy G

42,558

LaTeX commands are OK, but in book writing avoid such writing. For readability, it is preferably to write step-by-step calculations.. equations placed below each other better, I feel. – Dr. Sundar Mar 29 '22 at 07:57

score 2 · Answer 2 · answered Mar 29 '22 at 07:47

The plain TeX's \eqalign macro allows only two aligned columns, but OpTeX's \eqalign macro is more flexible and it allows arbitrary number of columns, see Typesetting Math with OpTeX, page 27. So, something like this is working:

\def\ralign#1{\left[\eqalign{#1}\right.}
$$
  \ralign{&x^2+2x-3(x+1)+3=0,&x+1\ge0\cr &x^2+2x-3(-x-1)+3=0,&x+1<0} \Rightarrow
  \ralign{&x^2-x=0,&x\ge-1\cr &x^2+5x+6=0,&x<-1} \Rightarrow
  \ralign{&x=1,x=0\cr &x=-2,x=-3}
$$
\bye

In pure Plain TeX, you must to define such extended \eqalign macro yourself using combination of \vcenter and \halign primitives.

score 2 · Answer 3 · answered Mar 29 '22 at 14:37

You can define your own environment modelled on cases:

\documentclass{article}
\usepackage{amsmath}
\newenvironment{orcases}
 {%
  \left\lbrack
  \renewcommand{\arraystretch}{1.2}%
  \array{@{}l@{\quad}l@{}}
 }
 {\endarray\right.}
\begin{document}
[
\begin{aligned}
\begin{orcases}
  x^2+2x-3(x+1)+3=0,  & x+1\ge0 \
  x^2+2x-3(-x-1)+3=0, & x+1<0
\end{orcases}
& \Rightarrow
\begin{orcases}
  x^2-x=0,    & x\ge-1 \
  x^2+5x+6=0, & x<-1
\end{orcases}
\
& \Rightarrow
\begin{orcases}
x=1,  x=0 \
x=-2, x=-3
\end{orcases}
\end{aligned}
]
\end{document}

I added a wrapping aligned, because the whole thing is too long for a single line.

How do I align a system of equations inside square brackets, for different cases?

3 Answers3