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Why does this not work in scrbook:

\[
\left | x \right |=\left\{\begin{matrix} x, if x\geq 0
\\ -x, if x<0 \end{matrix}\right.
\]

(I wrote it in writeLaTeX.)

if I use the amsmath then bring up a message:

"(no line number in this file):

LaTeX Error: Command \ iiint already defined.

Or name \ end ... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation. Type H for immediate help. ...

l.507 ... {\ iiint} {\ DOTSI \ protect \ MultiIntegral {3}}

Your command was ignored. Type I to replace it with another command, or to continue without it."

The codes I've used:

\documentclass[oneside]{scrbook} 
\usepackage{etoolbox} 
\usepackage{helvet} 
\usepackage[bahasa]{babel} 
\usepackage{calc} 
\usepackage[usenames,dvipsnames]{xcolor} 
\usepackage[utf8]{inputenc} 
\usepackage{chngcntr} 
\usepackage{graphicx} 
\usepackage{enumerate} 
\usepackage[inline,shortlabels]{enumitem} 
\usepackage{pifont} 
\usepackage{wasysym} 
\usepackage{sectsty} 
\usepackage{amssymb} 
\usepackage{array} 
\usepackage{venndiagram} 
\usepackage{tcolorbox} 
\usepackage{comment} 
\specialcomment{solusi}{\begin{tcolorbox}}{\end{tcolorbox}}
  • What doesn't work, exactly? You need \usepackage{amsmath} by the way. – Torbjørn T. Nov 7 '13 at 12:58
  • plus those entries in the matrix is text, so it is better to write x & \text{if $x\geq 0$} and -x & \text{if $x<0$} for the second line. – daleif Nov 7 '13 at 13:01
  • \documentclass[oneside]{scrbook} \usepackage{etoolbox} \usepackage{helvet} \usepackage[bahasa]{babel} \usepackage{calc} \usepackage[usenames,dvipsnames]{xcolor} \usepackage[utf8]{inputenc} \usepackage{chngcntr} \usepackage{graphicx} \usepackage{enumerate} \usepackage[inline,shortlabels]{enumitem} \usepackage{pifont} \usepackage{wasysym} \usepackage{sectsty} \usepackage{amssymb} \usepackage{array} \usepackage{venndiagram} \usepackage{tcolorbox} \usepackage{comment} \specialcomment{solusi}{\begin{tcolorbox}}{\end{tcolorbox}} – kalakay Nov 7 '13 at 13:07
  • @kalakay Please, add the code to your question rather than in comments. – egreg Nov 7 '13 at 13:14
  • 1
    Please update your question instead. Comments are not meant for massive amounts of code. Besides that code is not a full minimal example. Please post something can we can copy and compile as is, such that we do not have to add or guess anything. – daleif Nov 7 '13 at 13:14
3

You're missing a backslash character, \, between \left and {.

However, rather than just adding the missing backslash character, you may want to rewrite the entire expression to make use of the cases environment to obtain an overall better format.

enter image description here

\documentclass{scrbook}
\usepackage{amsmath}
\begin{document}
Original form, with backslash inserted after second \texttt{\textbackslash left}:
\[
\left| x \right|=
\left\{
  \begin{matrix} x, if x\geq 0\\ -x, if x<0 \end{matrix}
\right.
\]

\bigskip
Modified form, using \texttt{cases} environment:
\[
\left | x \right | = 
\begin{cases}
\hfill x & \text{if $x\geq 0$}\\
      -x & \text{if $x<0$}
\end{cases}
\] 
\end{document}
  • Nice trick with the \hfill. I've never thought of that in such a case. I didn't realize it would work in this context. – A.Ellett Nov 7 '13 at 13:22
  • Please note, I wrote it in writeLaTeX, if I use the amsmath then bring up a message: (no line number in this file): LaTeX Error: C ommand \ iiint already defined. Or name \ end ... illegal, see p.192 of the manual. See the LaTeX manual or LaTeX Companion for explanation. <return> Type H for immediate help. ... l.507 ... {\ iiint} {\ DOTSI \ protect \ MultiIntegral {3}} Your command was ignored. Type I <command> <return> to replace it with another command, or <return> to continue without it. – kalakay Nov 7 '13 at 13:24
  • @A.Ellett - Thanks. It works because the left-hand column of a cases environment is centered, by default, by placing \hfil at either end of the material. The \hfill instruction to the left of x serves to overpower the implicit \hfil that follows the x, and the cell ends up being typeset flush-right. :-) – Mico Nov 7 '13 at 13:26
  • @kalakay - I'm afraid I'm not familiar with writeLaTeX. Are there maybe other math-related packages you're loading? Have you tried deleting all .aux files and recompiling your document. – Mico Nov 7 '13 at 13:31
  • 1
    @kalakay if you are still having problems please edit your question (not comments) to have a complete document (as Mico did in this answer then people have a chance of explaining any error. – David Carlisle Nov 7 '13 at 14:02

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