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I want to have the solutions to some exercises in a table. I have succeeded in doing it just by using two arrays (and breaking up the equations) with the following code:

\documentclass{article}
\usepackage{amssymb,amsmath}

\begin{document}
\noindent \textbf{1.3 (4)}
$$\begin{array}{cccccc}
\textbf{(a)} & 13\equiv b \pmod{8} & \textbf{(b)} & 52\equiv b\pmod{9} & \textbf{(c)} & -8\equiv b\pmod{10}\\
& 13=8\cdot 1 +5 & & 52=9\cdot 5 +7 & & -8=10\cdot (-1) +2\\
& \therefore 13\bmod 8= 5 & & \therefore 52\bmod 9= 7 & & \therefore -8\bmod 10= 2 \\
& \therefore \text{ s{\"a}tt } b=5 & & \therefore \text{ s{\"a}tt } b=7 & & \therefore \text{ s{\"a}tt } b=7\\
\end{array}$$
$$\begin{array}{ccccccc}
\textbf{(d)} & 14\equiv b\pmod{14} & \textbf{(e)} & 8\equiv b\pmod{12} & \textbf{(f)} & -23\equiv b\pmod{11}\\
 & 14=14\cdot 1 +0 & & 8=12\cdot 0 +8 & & -23=11\cdot (-3) +10\\
 & \therefore 14\bmod 14= 0 & & \therefore 8\bmod 12= 8 & & \therefore -23\bmod 11= 10\\
 &\therefore \text{ s{\"a}tt } b=0 & & \therefore \text{ s{\"a}tt } b=8 & & \therefore \text{ s{\"a}tt } b=10\\
\end{array}$$
\end{document}

But I would like to know if it is possible to do it by embedding gathered equations into one array (as this makes more sense for me so I can keep the equations together instead of splitting them up). I tried the following code but can't get it to work:

\documentclass{article}
\usepackage{amssymb,amsmath}

\begin{document}
\noindent \textbf{1.3 (4)(a)}
$$\begin{array}{ccc}
\begin{gather*}
13\equiv b \pmod{8}\\
13=8\cdot 1 +5\\
\therefore 13\bmod 8= 5\\
\therefore \text{ s{\"a}tt } b=5\\
\end{gather*} &
\begin{gather*}
52\equiv b\pmod{9}\\
52=9\cdot 5 +7\\
\therefore 52\bmod 9= 7\\
\therefore \text{ s{\"a}tt } b=7\\
\end{gather*} &
\begin{gather*}
-8\equiv b\pmod{10}\\
-8=10\cdot (-1) +2\\
\therefore -8\bmod 10= 2\\
\therefore \text{ s{\"a}tt } b=7\\
\end{gather*}\\
\begin{gather*}
14\equiv b\pmod{14}\\
14=14\cdot 1 +0\\
\therefore 14\bmod 14= 0\\
\therefore \text{ s{\"a}tt } b=0\\
\end{gather*} &
\begin{gather*}
8\equiv b\pmod{12}\\
8=12\cdot 0 +8\\
\therefore 8\bmod 12= 8\\
\therefore \text{ s{\"a}tt } b=8\\
\end{gather*}&
\begin{gather*}
-23\equiv b\pmod{11}\\
-23=11\cdot (-3) +10\\
\therefore -23\bmod 11= 10\\
\therefore \text{ s{\"a}tt } b=10\\
\end{gather*}
\end{array}$$
\end{document}
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1 Answer 1

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your array is already in math mode, consequently you can not nested in it math mode again. Try:

\documentclass{article}
\usepackage{amssymb,amsmath}

\begin{document}
\noindent \textbf{1.3 (4)(a)}
\[\setlength\arraycolsep{12pt} % more space between columns
\begin{array}{ccc}
\begin{gathered}% it is designed for use inside math environment
13\equiv b \pmod{8}\\
13=8\cdot 1 +5\\
\therefore 13\bmod 8= 5\\
\therefore \text{ s{\"a}tt } b=5\\
\end{gathered} % <---
&   \begin{gathered} % <---
    52\equiv b\pmod{9}\\
    52=9\cdot 5 +7\\
    \therefore 52\bmod 9= 7\\
    \therefore \text{ s{\"a}tt } b=7\\
    \end{gathered} % <---
    &   \begin{gathered} % <---
        -8\equiv b\pmod{10}\\
        -8=10\cdot (-1) +2\\
        \therefore -8\bmod 10= 2\\
        \therefore \text{ s{\"a}tt } b=7\\
        \end{gathered} % <---                                  \\
% second row
\begin{gathered} % <---
14\equiv b\pmod{14}\\
14=14\cdot 1 +0\\
\therefore 14\bmod 14= 0\\
\therefore \text{ s{\"a}tt } b=0\\
\end{gathered} % <---
&    \begin{gathered} % <---
     8\equiv b\pmod{12}\\
     8=12\cdot 0 +8\\
     \therefore 8\bmod 12= 8\\
     \therefore \text{ s{\"a}tt } b=8\\
     \end{gathered} % <---
     &    \begin{gathered} % <---
          -23\equiv b\pmod{11}\\
          -23=11\cdot (-3) +10\\
          \therefore -23\bmod 11= 10\\
          \therefore \text{ s{\"a}tt } b=10\\
          \end{gathered} % <---
\end{array}
\]
\end{document}

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  • Thank you this is exactly the solution I was looking for! Commented Apr 14, 2017 at 10:20

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