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Hello.

I was trying to typeset the following system of equations:

system of equations

I used the following code in order to do it:

\begin{equation*}
    \left\{
    \begin{alignedat}{9}
        & b_{1,1} x_2 &&{} + {}&& b_{1,2} x_2 &&{} + {}&& \cdots &&{} + {}&& b_{1,9} x_9 &&{} = {}&& c_1 \\
        & b_{2,1} x_2 &&{} + {}&& b_{2,2} x_2 &&{} + {}&& \cdots &&{} + {}&& b_{2,9} x_9 &&{} = {}&& c_2 \\
        &&& \: \: \vdots &&&& \: \: \vdots &&&& \: \: \vdots &&&& \:\: \vdots \\
        & b_{9,1} x_2 &&{} + {}&& b_{9,2} x_2 &&{} + {}&& \cdots &&{} + {}&& b_{9,9} x_9 &&{} = {}&& c_9
    \end{alignedat}
    \right.
\end{equation*}

Now, I have 2 questions:

  1. Is there a more natural way to add the spacing between the columns?
  2. Why does {} + {} work (that is, adds the spaces around the + sign)? I got it by accident (desperate times call for desperate measures), but have no idea how/why it works.
d125q
  • 875
  • 8
  • 18
  • 1
    Welcome to TeX.SX! Why don't you use macros for spacing such as \quad or \qquad for add space? – Aradnix Sep 30 '14 at 22:05
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    As to the {}+{}, a plus sign can indicate an operation, as in a + b or it can be a sign as in +b. The spacing is different for these two cases. Adding the braces indicates to TeX that the plus should be taken as within an operation. – Steven B. Segletes Sep 30 '14 at 22:10

2 Answers2

9

The code {}+{} works because it adds empty atoms at either side of the operation, ensuring correct spacing.

However, you can input more easily this formula with array:

\documentclass{article}
\usepackage{amsmath}
\usepackage{array}

\begin{document}

\begin{equation*}
\left\{
\setlength{\arraycolsep}{0pt}% no padding
\newcolumntype{B}{>{{}}c<{{}}}
\begin{array}{ l B l B l B l B l }
b_{1,1} x_2 & + & b_{1,2} x_2 & + & \cdots & + & b_{1,9} x_9 & = & c_1 \\
b_{2,1} x_2 & + & b_{2,2} x_2 & + & \cdots & + & b_{2,9} x_9 & = & c_2 \\
  & \vdots && \vdots && \vdots &&  \vdots \\
b_{9,1} x_2 & + & b_{9,2} x_2 & + & \cdots & + & b_{9,9} x_9 & = & c_9
\end{array}
\right.
\end{equation*}

\end{document}

Here the {}+{} trick is used as well, but hidden in the temporary column type B (for Binary).

enter image description here

egreg
  • 1,121,712
8

You don't need so many alignment points. Actually, the number of alignment points essentially depends on the number of vertical dots you want to align. I give two possibilities, with 4 or 2 alignment points (7 or 3 ampersands). The alignment itself uses the \vdotswithin command, from mathtools, and the braces come from the empheq package (which loads mathtools). You don't need an alignedat environment nested in an equation*, alignat* will do the job.

    \documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage[overload]{empheq}


\begin{document}


\begin{alignat}{4}[left =\empheqlbrace]
     b_{1,1} x_2 &{}+ {}& b_{1,2} x_2 &{} + ⋯ +{} & b_{1,9} x_9 & ={} & c_1 \
 b_{2,1} x_2 &{}+{} & b_{2,2} x_2 &{} + ⋯ + {}& b_{2,9} x_9 & = & c_2 \[-1.5ex]
  \vdotswithin{b_{2,1} x_2}&& \vdotswithin{b_{2,2} x_2} && \vdotswithin{b_{2,9} x_9 } &&\vdotswithin{c_2} \[-1ex]
     b_{9,1} x_2 &{}+ {}& b_{9,2} x_2 &{} + ⋯ + {}& b_{9,9} x_9 & = & c_9
\end{alignat}


\begin{alignat}{2}[left =\empheqlbrace]
     b_{1,1} x_2 &{}+ b_{1,2} x_2 + ⋯ + b_{1,9} x_9 & ={} & c_1 \
 b_{2,1} x_2 &{}+ b_{2,2} x_2 + ⋯ + b_{2,9} x_9 & = {}& c_2 \[-1.5ex]
  \vdotswithin{b_{2,1} x_2} && \vdotswithin{ = {}} \[-1ex]
     b_{9,1} x_2 &{}+ b_{9,2} x_2 + ⋯ + b_{9,9} x_9 & = {}& c_9
\end{alignat}


\end{document}

enter image description here

Bernard
  • 271,350